Calc Dragon

A hoard of calculators, growing weekly.

We build calculators for the questions people actually type into a search box. Convert this into that. Work out the right amount. Decide whether the deal is a deal.

Every calculator on Calc Dragon implements a tested, correct formula and comes with a plain-English explanation of how the maths works. They cover finance, health, science, construction, maths, and everyday conversions — with new ones added most weeks.

Each result includes a worked example so you can check the logic yourself. These are educational tools: useful for getting a clear number quickly, but not a substitute for professional advice on decisions that really matter.

Currently in the hoard

Finance

401(k) Calculator — Project your 401(k) balance at retirement from salary, employee deferral, employer match, expected return, and years to retirement — with the 2025 IRS limits applied automatically.
403(b) Calculator — Project your 403(b) balance at retirement from salary, your elective deferral, your employer contribution, expected return and years to retirement — with 2025 IRS limits applied automatically.
50/30/20 Budget Calculator — Enter your monthly take-home income and a target split. The calculator divides your paycheck into needs, wants, and savings, and shows what each bucket is worth annually.
AGI Calculator — Work out your Adjusted Gross Income — the figure on IRS Form 1040 Line 11 — from your wages, other taxable income, IRA and HSA contributions, student loan interest and other above-the-line adjustments.
Alternative Minimum Tax (AMT) Calculator — Work out the extra federal tax you owe under the Alternative Minimum Tax on IRS Form 6251 — using the 2025 exemption, phaseout and 26%/28% TMT rates for your filing status.
Amortization Calculator — Work out the monthly payment on any fixed-rate loan and see how much of each payment goes to interest versus principal.
Annuity Calculator — Compute the future value, present value, or periodic payment of an annuity — the textbook time-value-of-money formulas used by Excel's FV(), PV(), and PMT() functions.
Annuity Future Value Calculator — Project the future value of regular payments — with or without a starting balance — using the standard ordinary-annuity and annuity-due formulas.
Annuity Payout Calculator — Work out the regular income a lump sum produces when annuitised at a given return rate over a fixed term — the textbook PMT formula used by Excel.
ARM Mortgage Calculator — Model the initial fixed payment and the payment shock at the first rate reset on a US hybrid ARM, with the lifetime cap applied.
Auto Lease Calculator — Estimate the monthly payment on a US car lease using the standard depreciation + finance charge + tax formula. Adjust vehicle price, residual, APR, term and tax to compare deals.
Average Return Calculator — Calculate the compound annual growth rate (CAGR) of an investment from its starting value, ending value, and the number of years held — the single number that summarises average annual performance with compounding.
Biweekly Mortgage Calculator — Compare a standard monthly mortgage with a biweekly payment plan and see how much time and interest you save.
Boat Loan Calculator — Estimate the monthly payment, total interest and true cost of a secured fixed-rate boat loan. Adjust the boat price, down payment, interest rate and term to compare offers.
Bond Price Calculator — Price any fixed-coupon bond from its coupon rate, term, and market yield. The result shows the clean price, current yield, total coupons over the holding period, and whether the bond trades at a premium or discount to par.
Business Loan Calculator — Estimate the monthly payment, total interest and origination fee on a fixed-rate business term loan, and see the true cost of borrowing once fees are included.
Buy-to-Let Mortgage Calculator — Work out your monthly payment, gross rental yield, interest coverage ratio (ICR), and LTV for a UK buy-to-let property.
Canadian Mortgage Calculator — Estimate the monthly payment and total interest on a Canadian fixed-rate mortgage, with the semi-annual compounding required by the federal Interest Act.
CD (Certificate of Deposit) Calculator — Project the maturity value of a US certificate of deposit at any APY and term. APY is the effective annual rate banks must disclose under Truth in Savings, so compounding frequency is already baked into the rate.
College Cost Calculator — Estimate the total cost of a future US college degree. Inflate today’s annual cost forward to the year of enrollment and sum across the years of attendance to see what the sticker price actually grows into.
Compound Interest Calculator — Project the future value of a deposit (with optional regular contributions) at any rate, term, and compounding frequency — the universal time-value-of-money formula.
Credit Card Payoff Calculator — Find out how long it takes to clear a credit card balance at a fixed monthly payment, and how much of every payment is interest versus principal.
Debt Avalanche Calculator — Model the avalanche payoff method — pay minimums on every debt, then send every spare pound or dollar at the highest-APR balance until it clears, then roll its payment into the next one.
Debt Snowball Calculator — Model the snowball payoff method — pay minimums on every debt, then send every spare pound or dollar at the smallest balance until it clears, then roll its payment into the next-smallest balance.
Debt-to-Income (DTI) Calculator — Work out the debt-to-income ratio lenders use when underwriting a mortgage, car loan, or refinance. Splits housing-only (front-end) from total (back-end) DTI and flags the band you fall in.
Debt-to-Income Ratio Calculator — Work out your back-end and front-end DTI from gross monthly income, housing payment, and other monthly debts. Uses the standard CFPB / Fannie Mae definition that mortgage underwriters apply.
Down Payment Calculator — Enter a home price and a deposit percentage to see the down payment, the loan amount you need to borrow, and the estimated monthly mortgage payment.
FHA Loan Calculator — Work out the full monthly cost of an FHA-backed mortgage — including the 1.75% upfront MIP and the monthly MIP that every FHA loan carries.
Future Value Calculator — Work out the future value of a present-day lump sum and a stream of regular payments at any nominal annual rate, term, and compounding frequency — the textbook time-value-of-money formula behind Excel's FV() function.
HELOC Calculator — Estimate the maximum home equity line of credit you could qualify for, and the interest-only monthly payment on a fully drawn line.
Home Equity Loan Calculator — Work out the fixed monthly payment, total interest and lifetime cost of a home equity loan — a lump-sum, fixed-rate second mortgage secured against your home.
House Affordability Calculator — Estimate the maximum home price you can afford in the US, using the 28/36 debt-to-income rule with a 30-year fixed mortgage and full PITI (principal, interest, taxes, insurance).
Inflation Calculator — Adjust an amount from one year to another at a chosen average annual inflation rate, with cumulative inflation and lost purchasing power shown alongside.
Interest Rate Calculator — Work out the annual interest rate on a fixed-rate amortising loan from the loan amount, the monthly payment, and the term in months — useful when the lender quotes a payment but not the underlying rate.
Investment Calculator — See how a starting balance and a steady monthly contribution can grow at a chosen rate of return, with both the nominal ending balance and the inflation-adjusted value in today's money.
IRA Calculator — Traditional vs Roth — Project the balance of a Traditional or Roth IRA at retirement, compare after-tax outcomes side-by-side, and see how the 2025 IRS contribution limits apply to you.
ISA Savings Calculator — Calculate how much your ISA could be worth over time with regular contributions and compound growth — and see exactly how much is tax-free gains.
Mortgage Affordability Calculator — Estimate how much a UK lender might let you borrow based on income, monthly outgoings, deposit, and the salary multiple your lender applies.
Mortgage Payoff Calculator — See how many years sooner you can pay off your mortgage and how much interest you save by adding a fixed extra amount to every monthly payment.
Mortgage Repayment Calculator — Work out exactly what your monthly mortgage payment will be — and how much interest you'll pay over the full term.
Mutual Fund Returns Calculator — Estimate the maturity value of a mutual fund investment — a starting lump sum plus a monthly SIP — at any expected annual return and time horizon. Uses the standard SIP future-value formula with monthly compounding.
P/E Ratio Calculator — Compute price-to-earnings ratio, earnings yield, and market capitalisation from share price and EPS. Uses the universal Brealey/Myers identity used in every equity-analysis textbook.
Pension Drawdown Calculator — Project how long your pension pot will last — or whether it will sustain your withdrawals indefinitely.
Personal Loan Calculator — Estimate the monthly payment, total interest and origination fee on a fixed-rate unsecured personal loan, and see the true cost of borrowing once fees are included.
Present Value Calculator — Discount a future lump sum and a stream of regular payments back to today's money at any nominal annual rate, term, and compounding frequency — the textbook time-value-of-money formula behind Excel's PV() function.
Refinance Calculator — Find out whether refinancing your mortgage actually pays off — and how many months until the lower payment covers the closing costs.
Rent Affordability Calculator — Work out how much rent you can comfortably afford from your gross monthly income using the 30% rule and the 40% debt-to-income cap most landlords screen against.
Rent vs Buy Calculator — Compare buying a home with renting on a like-for-like basis. The calculator equalises monthly cash flow, compounds the renter's alternative investment, and reports the break-even year where buying pulls ahead.
Required Minimum Distribution (RMD) Calculator — Calculate your IRS-required annual withdrawal from a traditional IRA, 401(k), 403(b), or other tax-deferred retirement account using the IRS Uniform Lifetime Table.
Retirement Calculator — Project the retirement nest egg you are on track to build, then convert it into a sustainable monthly income — with both nominal figures and an inflation-adjusted view in today's money.
ROI Calculator — Work out the total return on an investment, the net profit, and the annualised growth rate (CAGR) from your initial outlay, final value, and holding period.
Roth IRA Calculator — Project the tax-free balance of a Roth IRA at retirement, with 2025 IRS contribution limits and MAGI phase-out applied automatically by filing status.
Salary Calculator — Convert any pay rate into every other frequency in one step. Enter what you earn — hourly, weekly, monthly, or annual — and see the equivalent across all seven standard pay periods.
Sales Tax Calculator — Add US state-and-local sales tax to a pre-tax subtotal or extract the tax element from a tax-inclusive total. Enter the combined rate for the destination of the sale — typical US combined rates range from 0% to about 9.55%.
Savings Goal Calculator — Enter your savings target, your current balance, your monthly deposit, and the interest rate to see how many months it will take to reach the goal — solved with the standard time-value-of-money formula.
Student Loan Calculator — Enter your remaining loan balance, interest rate, and the monthly amount you can pay — the calculator returns payoff time, total interest, and how much you save by adding an extra fixed amount each month. Works for US federal/private and UK Plan-style loans on the repayment side.
UK Capital Gains Tax Calculator 2025/26 — Work out the capital gains tax you owe on shares, property, or other assets — split across the 18% and 24% bands using HMRC 2025/26 rules.
UK Dividend Tax Calculator 2025/26 — Find out exactly how much tax you owe on your dividend income — split across basic, higher, and additional rate bands — using the 2025/26 HMRC rates.
UK Stamp Duty Calculator 2025/26 — Calculate the Stamp Duty Land Tax owed on your property purchase using HMRC 2025/26 bands — including first-time buyer relief and the 5% additional-property surcharge.
UK Take-Home Pay Calculator 2025/26 — See exactly how much salary you keep after income tax and National Insurance, using the 2025/26 HMRC bands. Switch to Scottish rates if you live in Scotland.
US Federal Estate Tax Calculator 2026 — Estimate the federal estate tax owed on a US estate using the 2026 basic exclusion amount and the IRC § 2001 top rate.
VA Mortgage Calculator — Estimate the monthly payment, VA funding fee, and total cost of a US Department of Veterans Affairs home loan, with the funding fee financed into the balance.
VAT Calculator — Add VAT to a net price or strip VAT out of a gross price. Defaults to the UK 20% standard rate; switch to 5% reduced, 0% zero, or any custom rate for EU VAT, Australian GST, or other invoice-credit VAT systems.

Math

ANOVA Calculator — A one-way analysis of variance for up to five groups. Paste each group as a list of numbers and the calculator returns the F statistic, the degrees of freedom, the p-value, eta-squared, and the full SS / df / MS table.
Antilog Calculator — The antilogarithm is the inverse of the logarithm — antilog_b(x) = b^x. Enter an exponent x and a base; the calculator returns b^x plus 10^x, e^x and 2^x for reference.
Arithmetic Sequence Calculator — Enter the first term, the common difference, and how many terms you want. The calculator lists every term, the nth term aₙ, and the running total Sₙ.
Average Calculator — Paste or type a list of numbers — separated by commas, spaces or new lines — and the calculator returns the mean, median, mode and range, alongside the sum, count, minimum and maximum.
Big Number Calculator — Perform exact arithmetic on integers of any size — no rounding, no overflow. Powered by JavaScript BigInt for arbitrary-precision results.
Binomial Distribution Calculator — Enter the number of trials, the probability of success on each trial, and the number of successes you are interested in. The calculator returns the exact binomial probability P(X = k), every cumulative probability around k, and the mean and standard deviation of the distribution.
Chi-Square Test Calculator — A Pearson chi-square goodness-of-fit test on a single set of category counts. Paste observed counts and the calculator returns χ², degrees of freedom, the right-tailed p-value, and every cell’s contribution. Expected counts are optional — leave blank for a uniform distribution, or enter counts or ratios to test a specific hypothesis.
Confidence Interval Calculator — Enter your sample mean, standard deviation and sample size, choose a confidence level, and the calculator returns the confidence interval for the population mean along with the margin of error and standard error.
Distance Between Two Points Calculator — Find the straight-line (Euclidean) distance between any two points in 2D or 3D space. Enter the x, y, and z coordinates of each point — leave z at 0 for a 2D problem.
Exponent Calculator — Enter a base and an exponent and the calculator returns b raised to the power n. Works for positive, negative, integer and fractional exponents, with automatic scientific-notation output for extreme results.
Factor Calculator — Enter any whole number and the calculator lists every positive factor (divisor), its prime factorisation, all of its factor pairs, and σ(n) — the sum of its divisors — along with whether the number is prime, composite, or perfect.
Five-Number Summary Calculator — Paste or type a list of numbers and the calculator returns Tukey's five-number summary — minimum, Q1, median, Q3 and maximum — plus the interquartile range, Tukey fences and any outliers.
Fraction Calculator — Pick an operation, enter two fractions, and the calculator returns the exact answer in lowest terms — along with the decimal value, mixed-number form, and the unsimplified intermediate so you can follow the working.
GCF Calculator — Enter any list of whole numbers and the calculator returns the greatest common factor — the largest positive integer that divides every input exactly — along with its prime factorisation, all of its divisors, and the Euclidean reduction steps.
Geometric Mean Calculator — Paste or type any list of positive numbers. The calculator returns the geometric mean — the right average for growth rates, compounding returns, and ratios — alongside the arithmetic and harmonic means for comparison.
Geometric Sequence Calculator — Enter the first term, the common ratio, and how many terms you want. The calculator lists every term, the nth term aₙ, the partial sum Sₙ, and — when |r| < 1 — the infinite sum S∞.
Harmonic Mean Calculator — Paste or type any list of positive numbers. The calculator returns the harmonic mean — the right average for rates and ratios — alongside the geometric and arithmetic means for comparison.
Hex Calculator — Convert any number between hexadecimal, decimal, binary, and octal — all four representations side by side.
LCM Calculator — Enter any list of whole numbers and the calculator returns the lowest common multiple — the smallest positive integer that every input divides into exactly — along with the greatest common divisor and the reduction steps.
Logarithm Calculator — Pick a base (10, e, 2, or any positive number ≠ 1) and enter a positive argument x. The calculator returns log_b(x) plus the three standard logs alongside.
Number Sequence Calculator — Generate arithmetic and geometric sequences from a first term and a common difference or ratio. The calculator returns the nth term, the partial sum, the list of terms, and (where it exists) the infinite geometric sum.
P-Value Calculator — Convert a z, t or chi-squared test statistic into a p-value. Pick one- or two-tailed, enter your degrees of freedom (for t and χ²), and the calculator returns the exact p-value and whether the result is significant at α = 0.05 and α = 0.01.
Percent Error Calculator — Enter a measured (experimental) value and an actual (true) value to see the percent error — the standard metric used in physics, chemistry, and engineering labs to express how close a measurement is to the accepted value.
Quadratic Formula Calculator — Solve any quadratic equation ax² + bx + c = 0 for its real or complex roots, with the discriminant, vertex and axis of symmetry shown alongside.
Ratio Calculator — Enter two ratios A : B and C : D. The calculator simplifies each to lowest terms, shows the decimal and percentage values, tests whether the ratios are equivalent by cross-multiplication, and reports the scale factor between them.
Root Calculator — Enter a number and a root degree to compute the nth root. Square root (n=2), cube root (n=3), or any higher integer degree. Odd roots of negative numbers return a negative real result; even roots of negative numbers return a complex-number message.
Sample Size Calculator — Determine the minimum number of respondents needed to estimate a population proportion at a chosen confidence level and margin of error. Optional finite-population correction tightens the requirement when the population is small.
Standard Deviation Calculator — Paste or type a list of numbers and the calculator returns the standard deviation and variance, with a toggle between the sample (n − 1) and population (n) variants. Mean, sum of squared deviations and range are shown alongside.

Geometry

Acute Triangle Calculator — Type three side lengths and the calculator checks whether they form an acute triangle (every angle strictly less than 90°), reports all three angles via the law of cosines, and gives the area from Heron's formula.
Annulus (Ring) Area Calculator — Enter the outer and inner radii of a flat circular ring and the calculator returns the annulus area A = π(R² − r²), along with the ring width and the two bounding circumferences.
Area of an Oblique Triangle Calculator — An oblique triangle is any triangle without a right angle. Enter two sides and the angle between them; the calculator returns the area via ½·a·b·sin(C), plus the missing side and angles from the laws of cosines and sines.
Centroid of a Triangle Calculator — Enter the (x, y) coordinates of the three vertices. The calculator returns the centroid — the intersection point of the three medians — along with each side midpoint, the three median lengths, and the triangle area.
Circle Calculator — Pick which property of the circle you know — radius, diameter, circumference or area — type the value, and the calculator returns the other three using exact π.
Cone Volume Calculator — Type the base radius and height of a right circular cone — the calculator returns volume, slant height, lateral and total surface area, all using exact π.
Cylinder Volume Calculator — Type the radius and height of a right circular cylinder — the calculator returns volume, base area, lateral (curved) surface and total surface, all using exact π.
Ellipse Area Calculator — Type the two semi-axes of your ellipse and the calculator returns the area, an accurate perimeter approximation, the eccentricity and the focal distance.
Hexagon Calculator — Type the side length of a regular hexagon. The calculator returns the area, perimeter, apothem, long diagonal, width across the flats and circumradius — all from one input.
Pythagorean Theorem Calculator — Solve a² + b² = c² for any side of a right triangle, with area, perimeter and angles shown alongside.
Slope Calculator — Type the (x, y) coordinates of two points. The calculator returns the slope of the line through them plus the full line equation, midpoint, straight-line distance and the angle the line makes with the x-axis.
Sphere Volume Calculator — Type the radius of a sphere — the calculator returns volume, surface area, diameter, circumference and great-circle area, all using exact π.
Surface Area Calculator — Pick a solid — cube, sphere, cylinder, cone or square pyramid — enter its dimensions and get the total surface area, plus the lateral and base components, in any unit.
Tank Volume Calculator — Pick a tank shape — vertical cylinder, horizontal cylinder, or rectangular box — enter the dimensions and (optionally) a liquid depth. Get total volume, filled volume, head space and fill percent.
Triangle Calculator — Solve any triangle from three known measurements — sides, angles, area and perimeter, with the laws of sines and cosines doing the work behind the scenes.

Conversion

Acres to Hectares Converter — Convert between acres and hectares — bidirectional, exact, no rounding tricks. 1 acre = 0.40468564224 ha; 1 ha = 2.4710538147 acres.
Area Converter — Convert any area — metric or imperial. Square metres, square feet, acres, hectares, square miles, and the small units (cm², in², mm²) for design and engineering work.
Bandwidth & Download Time Calculator — Estimate how long it takes to transfer a file at a given connection speed. Handles bits vs bytes and decimal (MB) vs binary (MiB) units correctly.
Bar to PSI Converter — Convert between bar and pounds per square inch (psi) — bidirectional, exact, no rounding tricks. 1 bar = 14.5037737730 psi; 1 psi = 0.0689475729 bar.
Binary Calculator — Perform binary arithmetic and bitwise operations — add, subtract, multiply, divide, AND, OR, XOR — with results shown in binary, decimal, and hex.
Cooking Conversion Calculator — Convert any cooking volume — cups, tablespoons, teaspoons, fluid ounces, millilitres, pints — between US customary, UK imperial and metric (250 mL) cups, with a kitchen-equivalents breakdown.
Cups to Gallons Converter — Convert between cups and gallons — US, imperial, and metric. Exact NIST and UK statutory factors, bidirectional, no rounding tricks.
Distance & Length Converter — Convert between any distance or length unit instantly — metric, imperial, or both at once.
Force Converter — Convert any force — SI, metric, US customary or CGS. Newtons, kilonewtons, pound-force, ounce-force, kilogram-force, dynes, poundals, tonne-force.
Fuel Consumption Converter — Convert any fuel-economy reading: US mpg, Imperial mpg, litres per 100 km, or kilometres per litre. See all four at once, exact to NIST factors.
Horsepower Converter — Convert between mechanical horsepower (hp), metric horsepower (PS / ch / CV), electrical hp, boiler hp, watts, kilowatts, BTU per hour and foot-pounds per second.
IP Subnet Calculator (CIDR) — Enter an IPv4 address and a CIDR prefix (0–32). The calculator returns the network address, broadcast address, usable host range, subnet and wildcard masks, total addresses, host count, address class, and the IP and mask in binary.
Pressure Converter — Convert any pressure — SI, metric, US customary or vacuum/weather units. Pascals, bar, psi, atmospheres, mmHg, torr, inHg, ksi.
Roman Numeral Converter — Convert any whole number from 1 to 3999 into a Roman numeral, or any Roman numeral back into a decimal. Strict canonical form (MCMXCIV, not MDCCCCLXXXXIIII).
Shoe Size Converter — Convert any adult shoe size between US, UK, EU and Japanese (Mondopoint, cm) sizing. Pick your style (men’s/unisex or women’s), choose the region you know, and the calculator shows the equivalents in every other region plus an estimated foot length.
Speed Converter — Convert between any two units of speed — miles per hour, kilometres per hour, metres per second, knots, feet per second and more.
Temperature Converter — Convert any temperature — Celsius, Fahrenheit, Kelvin, Rankine, or Réaumur. Exact under the SI definitions; absolute zero is bounded.
Time Converter — Convert between any two units of time — milliseconds to millennia, with documented conventions for "month" and "year" so the answer is unambiguous.
Unit Converter — Convert between any two units in the same dimension — metric, imperial, US customary, and SI — with conversion factors that match NIST SP 811 and the 1959 International Yard and Pound Agreement.
Volume & Capacity Converter — Convert any volume — metric, US customary or UK imperial. Litres, gallons, fluid ounces, cups, cubic metres, even tablespoons and teaspoons.
Weight & Mass Converter — Convert between any weight or mass unit instantly — metric, imperial, US customary, or compound formats like stones and pounds.

Health

A Body Shape Index (ABSI) Calculator — Compute your A Body Shape Index (ABSI), the age- and sex-adjusted z-score, and the mortality-risk band — a measure of central adiposity that is statistically independent of BMI.
Adult Height Predictor — Estimate a child's likely adult height from the parents using the Tanner mid-parental method, with a 95% range so you can see how much the prediction varies.
Ankle-Brachial Index (ABI) Calculator — Compute the ankle-brachial index for each leg and classify peripheral artery disease severity using the AHA / ACC interpretation bands.
Apgar Score Calculator — Compute a newborn’s Apgar score from the five clinical components and classify it using the AAP/ACOG bands used in delivery rooms worldwide.
Army Body Fat Calculator — Estimate your body-fat percentage with the US Army tape test (Hodgdon-Beckett, DA Form 5500/5501) and see how it compares to the AR 600-9 maximum for your age.
BMI Calculator — Calculate your body mass index from weight and height and see where you sit on the World Health Organization adult BMI scale.
BMR Calculator — Estimate your basal metabolic rate — the calories your body burns at complete rest — using the Mifflin-St Jeor equation, the modern reference standard for healthy adults.
Body Fat Percentage Calculator — Estimate your body fat percentage from tape-measure circumferences using the U.S. Navy method — no calipers, no expensive scans.
Body Surface Area (BSA) Calculator — Enter your height and weight to get your body surface area in square metres, computed by the five major published formulas — Mosteller (clinical default), DuBois, Haycock, Gehan & George and Boyd.
Body Water Calculator — Estimate total body water using the clinical Watson (1980) equation, with a Hume–Weyers cross-check and the standard intracellular/extracellular compartment split.
Calorie Calculator — Enter your sex, age, weight, height and activity level. The calculator returns BMR, daily maintenance calories, a cutting target (-500 kcal) and a lean-bulk target (+300 kcal).
Calories Burned Calculator — Pick an activity, type your body weight and how long you trained. The calculator returns total kcal burned plus per-hour and per-minute rates using MET values from the published Compendium of Physical Activities.
Carbohydrate Intake Calculator — Enter your daily calories (TDEE) and a target carb percentage to get grams of carbohydrate per day. Defaults follow the Institute of Medicine’s Acceptable Macronutrient Distribution Range of 45-65 % of total calories from carbs.
Daily Water Intake Calculator — Estimate how much water you need each day based on body weight, exercise and climate, using the 35 mL/kg clinical baseline and ACSM exercise additions.
Fat Intake Calculator — Enter your daily calories (TDEE) and a target fat percentage to get grams of dietary fat per day. Defaults follow the Institute of Medicine’s Acceptable Macronutrient Distribution Range of 20-35 % of total calories from fat.
Healthy Weight Calculator — Find the weight range that puts you in the World Health Organization healthy BMI band for your height, in kilograms or pounds.
Lean Body Mass Calculator — Estimate your lean body mass using three published equations — Boer, James and Hume — and see the implied body fat percentage.
Marathon Pace Calculator — Enter a target marathon finish time to see the average pace per kilometre and per mile you would need to hit, plus an even-effort split table at every 5K and the half-marathon mark.
Marathon Time Predictor — Enter a recent race time and distance to predict your marathon finish time using Riegel's endurance formula. Works from a 5K, 10K, 15K, half marathon and other common race distances.
Overweight Calculator — Find out how far above the World Health Organization healthy BMI band you are, in kilograms or pounds.
Period Calculator — Predict your next period, ovulation day and fertile window from the first day of your last period and your usual cycle length.
Pregnancy Due Date Calculator — Estimate your estimated due date (EDD) and trimester milestones using Naegele’s rule — the 40-week (280-day) standard adopted by ACOG and used by midwives and obstetricians worldwide.
Pregnancy Weight Gain Calculator — Compare your weight gain so far against the Institute of Medicine recommended range for your pre-pregnancy BMI and pregnancy week.
Protein Calculator — Enter your body weight (kg or lb) and pick an activity level to get your daily protein target in grams. Targets follow the Institute of Medicine RDA for sedentary adults and the ACSM, Academy of Nutrition and Dietetics, Dietitians of Canada and ISSN positions for athletes.
Running Pace Calculator — Enter the distance and time of any run to see your pace per kilometre and per mile, speed in km/h and mph, and projected finish times at the standard race distances — 5K, 10K, half marathon and marathon.
Sleep Calculator — Pick whether you want to wake up at a target time or go to bed at a target time. The calculator works backward or forward in 90-minute sleep cycles so you wake at the end of a cycle — not stuck in deep sleep.
TDEE Calculator — Estimate the calories you burn in a typical day. The calculator splits TDEE into resting metabolism (BMR) plus activity burn so you can see where the kcal go.

Time And Date

Age Calculator — Work out exact age in years, months, and days between a date of birth and any reference date. Also shows total days lived, day of the week born, and the next birthday.
Age Difference Calculator — Enter two dates of birth and get the exact age gap in years, months, and days — plus the total days, weeks, and months between them.
Age in Weeks Calculator — Work out how many weeks old you are. Enter a date of birth and a reference date to see total weeks lived, the day remainder, total days, and the day of the week you were born.
Age on Other Planets Calculator — Enter your Earth age and see how old you would be on every other planet in the Solar System — in local planet-years and in local sidereal days (sols).
Countdown Calculator — Count the days, weeks, hours, minutes, and seconds remaining until any target date — or how long it has been if the date is already past.
Date Calculator — Add or subtract days, months, and years from any date. Returns the resulting date in long form and ISO 8601, the day of the week, and the total day shift.
Day Counter — Count the exact number of days between two dates — and see the same span as years/months/days and as weeks. Works for past, future, or mixed.
Days Until Valentine's Day Calculator — Count down to 14 February — the days, weeks, hours, minutes, and seconds remaining until Valentine's Day, plus the day of the week it falls on.
Time Duration Calculator — Find the elapsed time between two times of day. Returns hours and minutes, decimal hours, total minutes, and flags whether the duration crosses midnight.
Workday Calculator — Count the number of working days — Monday to Friday — between any two dates. Optionally exclude a number of public holidays. Useful for project deadlines, contract terms, payroll periods, and statutory time limits.

Everyday

3x Rent Rule Calculator — Most US landlords require gross monthly income of at least three times the rent. This calculator works it both ways: max rent for your income, and required income for a target rent.
Acceptance Rate Calculator — Work out what percentage of applicants get in. Enter the number who applied and the number accepted to see the acceptance rate, rejection rate, and Carnegie selectivity band — plus the optional yield rate if you know how many enrolled.
Alcohol By Volume (ABV) Calculator — Work out the alcohol by volume of your beer, wine, cider or mead from a pair of specific-gravity readings — using both the popular simple formula and the more accurate advanced (Hall) formula used for high-gravity brews.
Alcohol Units Calculator — Work out UK alcohol units from any drink — pint, glass of wine, spirit, cocktail — using the official NHS formula, and see how it compares to the Chief Medical Officers’ 14-unit weekly low-risk guideline.
Aspect Ratio Calculator — Enter a ratio and one dimension to compute the matching width or height in pixels — plus the simplified ratio and the common name (HD 16:9, photo 3:2, cinema, ultrawide).
BAC Calculator — Estimate your blood alcohol content from your weight, biological sex, number of standard drinks, and hours since your first drink. Uses the Widmark formula — the same model used in forensic toxicology. Educational only: never use this to decide whether to drive.
BBQ Party Calculator — Tell us who is coming and how long the party runs. We work out the meat to buy raw, the cooked portion per guest, sides, buns, drinks and ice.
Colour Mixer Calculator — Enter two hex colours and a blend share to get the mixed colour as hex, RGB, and HSL — using the same linear sRGB formula as the CSS color-mix() function.
Dice Roller — Roll any number of dice — d4 to d100 — with an optional modifier. Change any input to reroll.
Discount Calculator — Enter the original price and the discount % to see the sale price and what you save. Useful for shop-window quick-checks, voucher codes and stacked promotions.
Fuel Cost Calculator — Enter the trip distance, your fuel economy, and the pump price. Get the total cost, fuel used in litres and gallons, and cost per mile or km.
Gas Mileage Calculator — Enter how far you drove and how much fuel you used. Get your real-world economy in US mpg, UK mpg, L/100km and km/L — useful for cross-checking the dashboard reading or comparing cars across markets.
Golf Handicap Calculator — Convert your Handicap Index into a Course Handicap for any tee using the WHS formula adopted by the USGA and the R&A.
Mileage Reimbursement Calculator — Multiply your business-travel distance by an agreed rate per mile or kilometre to total reimbursement owed. Works for IRS standard mileage in the US, HMRC AMAP rates in the UK, CRA per-kilometre allowances in Canada, or any custom employer policy — enter your own rate.
Pizza Calculator — Tell us how many adults and kids are coming and the pizza size, and we will work out how many pizzas to order using the standard slice-per-guest rule.
Snow Day Calculator — Predict the probability of a school closure tomorrow from forecast snowfall, overnight low, wind speed, and where you live. Treat the result as a guide, not a forecast.
Streaming Services Cost Calculator — Add up everything you stream and find out what those subscriptions really cost you over 5, 10, or 20 years — including annual price hikes and the investment returns you give up.
Subscription Cost Calculator — Turn any recurring subscription into a daily, weekly, monthly, annual and lifetime cost — and see what optional price increases compound to over time.
Tip Calculator — Enter the bill, the tip % and how many people are splitting it. The calculator returns the tip, the total bill and what each person pays — useful for restaurants, bars, taxis and group meals.
Tire Size Calculator — Convert a tire designation (e.g. 205/55R16) into real-world geometry and compare two sizes to see the speedometer error if you switch.
Typing Speed Calculator — Convert a number of characters typed in a given time into words per minute. Net WPM penalises uncorrected errors at the standard rate of one word per error.

Construction

AC Tonnage Calculator — Estimate the central air-conditioning size your home needs. Enter conditioned square footage, climate zone and a few envelope details, and get the cooling load in BTU/hr and the recommended unit size in tons.
Asphalt Calculator — Enter your paving area dimensions and we work out the asphalt tonnage, volume (m³ and yd³), and total cost based on the standard hot-mix density.
Beam Deflection Calculator — Estimate the maximum deflection and bending moment of a prismatic beam. Pick the support and load case, set the span, load, Young's modulus and second moment of area, and the calculator returns the textbook closed-form answer.
Board Foot Calculator — Enter the nominal thickness, width and length of a piece of lumber — we calculate the board feet per piece, the total board feet for the order, and the cost at your lumber yard price.
Boiler Size Calculator — Estimate the right kW output for a new boiler from your floor area, insulation level, boiler type and number of bathrooms — the same rule of thumb used by Energy Saving Trust and the major UK boiler manufacturers.
Brick Calculator — Enter your wall length and height, pick a brick size, and the calculator returns the number of bricks to order (including a waste allowance) and the total material cost — works for UK and US brick formats.
BTU Calculator — Estimate the cooling capacity (BTU/hr) your room needs from an air conditioner. Uses the ENERGY STAR sizing rule of 20 BTU per square foot, with adjustments for sun, occupants, and kitchens.
Carpet Calculator — Enter your room dimensions and pick a carpet roll width — the calculator returns the number of drops, linear metres of roll, m² of carpet to order, gripper rod perimeter, and total cost. Works for UK 4 m / 5 m rolls and US 12 ft / 15 ft rolls.
Concrete Calculator — Enter your slab or column dimensions and we work out the concrete volume (m³ and yd³), the number of bags you need, and the total cost.
Concrete Driveway Cost Calculator — Estimate what a poured concrete driveway will cost. Enter the dimensions, slab thickness, concrete price, labor rate and reinforcement, and get total cost, concrete volume in cubic yards and installed cost per square foot.
Decking Calculator — Enter the deck size, board width and joist spacing — we work out the board count, joist count, linear metres of timber and the approximate screws you need.
Drywall Calculator — Enter your room dimensions and pick a sheet size — the calculator returns the number of drywall sheets to order (including waste), joint compound buckets, screws, and total material cost. Works for UK plasterboard and US gypsum board.
Fence Calculator — Enter your total fence length, post spacing and rails per section and we work out the posts, linear rail metres, and material cost — works for timber, post-and-rail, or closeboard fencing.
Roof Pitch Calculator — Enter the rise and run of your roof to get the pitch in the standard "X-in-12" notation, the slope angle in degrees, the percent grade, and the rafter length — all from one ratio.
Square Footage Calculator — Enter your shape and dimensions and we work out the area in square feet, square metres and square yards — and the total cost if you add a rate per ft².
Stair Calculator — Enter the floor-to-floor rise, your maximum riser height, and the tread depth (going). The calculator returns how many risers you need, the exact riser height, the total horizontal run, the stringer length, and the pitch angle — with a Blondel-formula comfort check.
Tile Calculator — Enter your room and tile dimensions and we calculate tiles needed (with wastage), boxes to order and total cost. Mix metric and imperial freely.

Science

Activation Energy Calculator — Enter two rate constants measured at two temperatures and this calculator returns the activation energy Eₐ using the two-point Arrhenius equation, with the pre-exponential factor A shown alongside for sanity checks.
Air Density Calculator — Work out the density of moist air from temperature, pressure and relative humidity. Returns the dry-air and water-vapour contributions separately so you can sanity-check the result.
Capacitor Energy Calculator — Enter any two of capacitance, voltage and stored energy — the calculator returns the third using E = ½ C V², and also reports the charge Q = C·V on the plates.
Dew Point Calculator — Enter the current air temperature and relative humidity. The calculator returns the dew point — the temperature at which the air would have to cool for water to start condensing — plus the temperature–dew-point spread and a perceived-comfort band.
Dilution Calculator (C₁V₁ = C₂V₂) — Work out how much stock to take, how much solvent to add, or what concentration you will end up at when diluting a solution. Solves any of the four variables in C₁V₁ = C₂V₂.
Engine Horsepower Calculator — Compute engine horsepower three ways: from a dyno torque curve (HP = T × RPM / 5252), from quarter-mile trap speed (Huntington method), and from quarter-mile elapsed time (Patrick Hale method).
Escape Velocity Calculator — Work out the minimum surface speed needed to leave a body’s gravity for ever, given the body’s mass and radius. Pick a preset (Earth, Moon, Mars, the Sun…) or enter a custom mass and radius.
Force Calculator — Enter any two of force, mass and acceleration — the calculator returns the third using Newton’s second law, F = m·a, and converts the force into common units.
Half-Life Calculator — Solve any of N₀, N(t), t, or T from the other three using the standard decay equation N(t) = N₀ × (½)^(t/T). Works for radioactive isotopes, first-order chemical reactions, and drug elimination — any process with a constant half-life.
Heat Index Calculator — Enter the air temperature and relative humidity. The calculator returns the heat index — what hot, humid air actually feels like on exposed skin — using the National Weather Service Rothfusz regression, plus an NWS heat-risk band.
Mass Calculator — Enter density and volume in your preferred units. The calculator returns the mass in kilograms, grams, pounds and ounces using m = ρ · V.
Molarity Calculator — Compute the molarity of a solution, the mass of solute needed for a target concentration, or the volume to make from a given amount — all from M = n/V with n = mass/MW.
Molecular Weight Calculator — Type a chemical formula and get the molar mass in g/mol, with each element’s mass contribution and percentage.
Ohm's Law Calculator — Enter any two of voltage (V), current (I), resistance (R) or power (P). The calculator returns the other two using Ohm's law and the DC power equations.
pH Calculator — Enter any one of [H⁺], [OH⁻], pH or pOH — the calculator returns the other three at 25 °C, plus an acid/base classification.
Resistor Color Code Calculator — Pick the four band colours and read out the resistance in ohms, kilo-ohms or mega-ohms — with the tolerance min/max range — using the IEC 60062 colour code.
Voltage Drop Calculator — Calculate voltage drop across a copper or aluminum branch circuit or feeder. Uses NEC Chapter 9 Table 8 DC resistance at 75 °C and flags whether the run meets the NEC-recommended ≤ 3 % branch-circuit limit.
Wavelength Calculator — Enter any two of wavelength, frequency and wave speed — the calculator returns the third using the universal wave relation v = f·λ, with results converted into the SI prefixes that make sense at that scale.
Wind Chill Calculator — Enter the air temperature and wind speed. The calculator returns the wind chill — what the cold actually feels like on exposed skin — using the National Weather Service and Environment Canada formula, plus a frostbite risk band.
Wire Gauge Calculator — Look up the ampacity, diameter, cross-section, and DC resistance of an American Wire Gauge conductor. Covers 14 AWG to 4/0 in copper and aluminum at 60, 75, and 90 °C insulation ratings, with the NEC Table 310.16 ampacity and the NEC 240.4(D) small-conductor overcurrent cap applied automatically.
Young's Modulus Calculator — Enter the axial force, cross-sectional area, original length and elongation of a specimen. The calculator returns stress σ, strain ε and Young's modulus E.

Business

Accounts Payable Turnover Calculator — Work out how many times a business pays off its supplier balances in a period, plus the equivalent days payable outstanding. Useful for cash-flow analysis, vendor relations, and lender ratio checks.
Accounts Receivable Turnover Calculator — Work out how many times a business collects its average receivables in a period, plus the equivalent days sales outstanding. Useful for cash-flow analysis, credit-policy review, and lender ratio checks.
Asset Turnover Ratio Calculator — Measure how efficiently a company converts its asset base into revenue. Computes asset turnover, average total assets, and capital intensity from a single year of figures.
Cap Rate Calculator — Work out the capitalization rate — the unlevered annual yield — on any rental property by entering its value, gross rent, vacancy rate, and operating expenses. Industry-standard real-estate valuation maths used by appraisers and investors.
Churn Rate Calculator — Calculate customer and revenue churn, retention rate, annualised churn, and the average customer lifetime implied by the periodic churn rate. Standard SaaS metric used by VC firms and growth teams.
Commission Calculator — Calculate sales commission, total earnings, and the variable share of pay from a sales volume, commission rate, and optional base salary. Works for straight commission, salary-plus-commission, and tiered plans (run the tier separately).
Contribution Margin Calculator — Calculate contribution margin per unit, contribution margin ratio, break-even volume, and net operating income from selling price, variable cost per unit, units sold, and fixed costs. Uses the standard cost-volume-profit (CVP) formula taught in every managerial accounting textbook.
Customer Acquisition Cost (CAC) Calculator — Calculate customer acquisition cost, payback period, and the LTV needed to clear a 3:1 ratio. Standard SaaS unit-economics formula used by VC firms and growth teams.
EBITDA Calculator — Calculate EBITDA — Earnings Before Interest, Taxes, Depreciation, and Amortization — and EBITDA margin from a company income statement. Uses the SEC Regulation G bottom-up formula.
EBITDA Margin Calculator — Calculate EBITDA margin — cash operating profit as a percentage of revenue — from a company income statement. Uses the top-down formula EBITDA = EBIT + Depreciation + Amortization.
Enterprise Value Calculator — Calculate enterprise value (EV) — the takeover value of a business, capital-structure-neutral — using the standard EV = Market Cap + Debt + Preferred + Minority − Cash identity.
GDP Calculator — Compute Gross Domestic Product from its expenditure components — consumption, investment, government spending and net exports. Add a GDP deflator for real GDP and a population for per-capita output.
Gross Margin Calculator — Calculate gross profit, gross margin percentage, and markup from revenue and cost of goods sold. Uses the universal accounting identity used in GAAP, IFRS, and UK FRS 102 reporting.
Inventory Turnover Calculator — Measure how many times a year inventory cycles through sales — and how many days, on average, a unit sits on the shelf before it ships. Computes turnover, average inventory, and DIO from a single year of figures.
IRR (Internal Rate of Return) Calculator — Find the discount rate that makes a project break even — the IRR. Universal corporate-finance formula used in capital budgeting, project appraisal, and private-investment screening.
Marginal Cost Calculator — Calculate marginal cost per unit, the change in total cost, and average cost at two production levels using the standard microeconomics identity MC = ΔTC / ΔQ. The same formula used in producer theory, managerial accounting, and pricing decisions.
Markup Calculator — Calculate selling price, gross profit, and gross margin from a cost and a markup percentage. Uses the universal retail pricing identity: Price = Cost × (1 + Markup).
NPV (Net Present Value) Calculator — Calculate the net present value of an investment by discounting future cash flows at your cost of capital. Universal corporate-finance formula used in capital budgeting, project appraisal, and DCF valuation.
Operating Margin Calculator — Measure how much of each dollar of revenue survives after the cost of producing the goods and running the business. Computes operating income, operating margin, gross profit, and gross margin from three income-statement figures.
Payback Period Calculator — Find the number of years until cumulative cash flows recover an upfront investment — the classic liquidity check used in capital budgeting alongside NPV and IRR.

New ones land most weeks. If there's a calculation you keep doing by hand, we probably haven't built it yet — but we will.