Calculators

Twenty-five tools across four everyday families. Knowing which family you are in tells you how much to trust the answer — a compound-interest figure is exact arithmetic on the numbers you give it, while a body-fat estimate is a published formula applied to a tape measure. Unit, number and colour conversion now lives in its own Converters hub, and passwords, QR codes and random names in the Generators hub.

Money: short formulas, unexamined assumptions

The equal monthly payment on a fully amortising loan is principal × r × (1+r)ⁿ ÷ ((1+r)ⁿ − 1), where r is the annual rate divided by twelve and n is the number of months. Loan EMI Calculator runs that and produces the year-by-year amortisation table, which is the informative part: on a long loan the early payments are overwhelmingly interest, and the balance barely moves for years. At 0% the formula degenerates to principal divided by months.

Compound Interest Calculator and Simple Interest Calculator are worth seeing side by side, because the gap between them is what compounding actually means. Simple interest is linear — the same amount added every year. Compound interest grows the base each period, and how often it compounds changes the answer: 8% compounded monthly beats 8% compounded annually. Regular contributions are handled as an ordinary annuity, which is how savings and pension projections work.

Two arithmetic traps recur in the shopping tools. Stacked discounts do not add: 20% off followed by a further 10% off is 28% off the original, not 30%, and Discount Calculator shows the running total rather than the sum of the percentages. And removing tax from a price that already includes it is division, not multiplication — Sales Tax Calculator does both directions, because getting that one backwards overstates the tax by a visible margin.

Tip Calculator splits a bill between people, Salary Calculator converts between hourly and annual pay, and ROI Calculator reports gain over cost as a percentage. The salary one hides the only real assumption: annual pay divided by 2,080 hours presumes 40 hours for 52 weeks, and anyone with unpaid leave or overtime will get a different true hourly rate. ROI is a ratio with no time dimension, so 20% over five years and 20% over five months display identically and are nothing alike. Percentage Calculator covers the underlying arithmetic all three lean on.

Health numbers are estimates, and the formulas are published

BMI Calculator divides kilograms by metres squared and classifies the result against the World Health Organization bands, with the healthy window at 18.5 to 24.9 and the weight range for your height derived from it. Its weakness is structural: it cannot distinguish muscle from fat, so it overstates risk for athletic builds and misses fat carried centrally. It is a population screening measure being applied to one person.

BMR Calculator uses Mifflin-St Jeor — ten times weight in kilograms, plus 6.25 times height in centimetres, minus five times age, then plus five for men or minus 161 for women — which is the most widely recommended equation, with the revised Harris-Benedict available for comparison. They disagree by around five percent on the same person, which is a fair indication of how precise any of this is.

TDEE Calculator multiplies that by an activity factor running from 1.2 for sedentary to 1.9 for hard training or a physical job. The factor is the least reliable input in the entire calculation, because nearly everyone overestimates their own activity. Macro Calculator then splits the calorie figure across protein, carbohydrate and fat at four, four and nine calories per gram.

Body Fat Calculator implements the US Navy circumference method from neck, waist, hip and height, classified against American Council on Exercise bands. A few percentage points of disagreement with hydrostatic weighing is normal for any tape-measure method. Ideal Weight Calculator deliberately shows four published formulas — Devine, Robinson, Miller and Hamwi — because they disagree by several kilograms at the same height, and printing one number would be false precision.

Water Intake Calculator works from roughly 35 ml per kilogram of body weight plus about 350 ml for every half hour of exercise. Target Heart Rate Calculator estimates maximum heart rate with Tanaka's formula, 208 minus 0.7 times age, which tracks measured values better than the familiar 220 minus age — particularly past forty — and converts it into training zones, with Karvonen targets if you add a resting rate.

Pregnancy Due Date Calculator applies Naegele's rule: 280 days from the first day of the last period, shifted by however much your cycle differs from 28 days. It is an estimate from dates rather than a scan, and only about five percent of babies arrive on the calculated day.

Mathematics, done exactly where it can be

Fraction Calculator works in integers throughout, reducing with Euclid's algorithm, so a third plus a sixth is exactly a half with no floating-point residue and the sign always sits on the numerator. Decimal to Fraction Converter operates on the string you typed rather than the parsed number, which is why 0.75 comes back as three quarters and not as the 3377699720527855 over 4503599627370496 that binary floating point would suggest. A repeating decimal is converted as the finite value entered — 0.333 becomes 333/1000, not one third — and the tool says so.

GCF and LCM Calculator shows the prime factorisation behind both, computing the LCM as one number divided by the GCD then multiplied by the other, an ordering that avoids overflowing on large inputs. Ratio Calculator simplifies and scales.

Average Calculator reports mean, median and mode together, because they answer different questions and only one of them is dragged around by a single outlier. When nothing repeats there is genuinely no mode, and the tool reports that rather than claiming every value is one. Standard Deviation Calculator distinguishes the sample figure, which divides by n minus one, from the population figure, which divides by n — the correction matters most on small samples, and the sample form needs at least two values to mean anything.

Number Base Converter handles binary, octal, decimal and hexadecimal, tolerating a leading 0x and digit separators, for non-negative integers. Roman Numeral Converter covers 1 to 3,999 and validates by round-tripping through the canonical form, so IIII and VX are rejected even though they would otherwise parse.

Dates, and the average month

Age Calculator and Date Difference Calculator work in UTC internally, so a span is not off by one day because a daylight-saving transition fell inside it. Timezone Converter handles the offset question directly, and Days to Hours, Hours to Minutes and Minutes to Hours cover the simple end.

One caveat applies to every duration tool: a month is not a fixed length. Where a month or a year has to be treated as a duration, the values used are the averages — 2,629,800 seconds and 31,557,600 — because calendar months run from 28 to 31 days. Anything claiming thirty days equals one month is approximating, and over a few years the error is visible.

Precision, and why the last digit sometimes looks wrong

JavaScript numbers are IEEE-754 double-precision floats, so 0.1 plus 0.2 is 0.30000000000000004. These calculators round at the display boundary and use exact integer or string arithmetic wherever it matters, but the underlying representation is still binary floating point. Results above roughly ten to the twelfth power or below ten to the minus sixth switch to exponential notation, because a full decimal expansion of either is unreadable. If two calculators disagree in the sixth decimal place, you are almost certainly looking at representation residue rather than two different answers.

None of it leaves the page

Every calculation runs in your own browser. That matters more here than anywhere else on the site, because the inputs are salaries, loan balances, body measurements and medical dates — the numbers least worth handing to a stranger's server. Nothing is transmitted, nothing is stored, and the tools keep working with the network disconnected.