Fitness Calculators

Body mass index from your height and weight, with the standard WHO adult categories. For how BMI works, its limitations, and the FAQ, see the full BMI Calculator page.

Daily calories to maintain, lose, or gain weight, from the Mifflin-St Jeor equation and your activity level. For how the targets are derived, accuracy notes, and the FAQ, see the full Calorie Calculator page.

Not medical advice: these calculators provide general educational estimates from published population formulas. Consult a qualified healthcare professional before changing diet or exercise. All data is processed locally in your browser and never transmitted.

Estimates from published formulas, not measurements of you

Every tool here implements an equation that was fitted to measurements of some study population. That distinction matters more than it sounds: the output is a population-level estimate applied to you, not an observation of you. A calorie figure from the Mifflin-St Jeor equation is not calorimetry; a tape-measure body-fat percentage is not underwater weighing; an age-predicted maximum heart rate is not a supervised exercise test. Each calculator page names its formula, says where it came from, and states the error band the literature reports for it — roughly ±10% around measured resting expenditure for Mifflin-St Jeor, roughly 3–4 percentage points for the US Navy circumference method, a spread of ±10–15 bpm for the 220 − age convention.

The practical consequence is that a single reading rarely carries much information. Taken with consistent technique, repeated readings are more informative than any single one — which is why the calorie target is framed as a two-to-three-week experiment against the actual weight trend rather than as a fixed prescription. Compared against yesterday's reading to one decimal place, these numbers are mostly noise. That is also why results are shown at the precision the estimates actually have — calories and grams as whole numbers, BMI and body fat to one decimal — rather than at the precision a computer could happily print.

BMR, TDEE, and calorie targets: one chain, three tools

The three energy calculators are consecutive steps in the same calculation, which is why it is often unclear which one you want. BMR is the base — the energy the body uses at complete rest. For the reference person above, a 25-year-old man of 70 kg and 175 cm, Mifflin-St Jeor gives 1,674 kcal/day. TDEE multiplies that base by an activity factor to estimate the whole day: 2,009 kcal/day at the sedentary factor of 1.2, and 2,594 kcal/day at the moderately active 1.55. The Calorie Calculator takes the last step, offsetting that maintenance figure into loss and gain targets — 2,094 kcal/day for the faster of its two loss options in the same example.

Notice where the uncertainty actually enters. The equation is fixed and the arithmetic is exact; the activity multiplier is a self-assessment, and here the difference between the sedentary and moderately active factors alone is 586 kcal/day. It is a judgement most people get wrong in the optimistic direction, which is the honest reason to treat any calorie target as the starting point of a two-to-three-week experiment against your actual weight trend rather than as a prescription. The engine flags rather than hides loss targets that fall below the estimated BMR or below commonly cited minimum intakes — about 1,200 kcal/day for women and 1,500 for men — because those are circumstances for qualified medical supervision, not for a calculator.

BMI, body fat, and ideal weight answer different questions

These three look interchangeable and are not. BMI is a ratio of mass to height squared: the same 70 kg, 175 cm person reads 22.9, which the WHO adult bands classify as normal weight. It is a screening ratio rather than a body-fat measure, and its blind spots are well documented — muscular people read high while carrying little fat, older adults can read normal while carrying little muscle, it is not meaningful during pregnancy, and the adult cut-offs do not apply to children and teens at all, where practice uses age-and-sex percentile charts instead.

The Body Fat Calculator asks a different question — what share of that mass is fat — and answers it from girths rather than from weight at all. At 175 cm with a 37 cm neck and an 85 cm waist, the US Navy circumference method estimates 17.7%; change nothing but the tape position and tension and the result can move by a point or more, which is why consistent technique matters more than any single figure. Ideal weight asks a third question and answers it four ways: for a man of 175 cm, the classic Devine, Robinson, Miller, and Hamwi formulas span 68.7–72 kg. The spread is the point. They were fitted to different reference data for different purposes — the Devine formula was proposed for drug dosing — so reading the range is more honest than crowning a winner, and none of the four consider body composition.

Target heart rate or zone 2?

Both start from the same estimate of maximum heart rate and differ only in how much of the range they show. The Target Heart Rate Calculator builds the full 50–85% ladder: at age 30 the age-predicted maximum is 190 bpm, and the rungs run from 95 bpm at 50% to 162 bpm at 85%. The Zone 2 Calculator zooms in on the easy-aerobic 60–70% band, which, with no resting heart rate entered, is exactly the 60% and 70% rungs of that percent-of-maximum ladder — 114–133 bpm — and, when you add a resting heart rate, shows the Karvonen version beside it rather than averaging the two conventions together. With a resting rate of 60 the same percentages become 138–151 bpm, because Karvonen scales the reserve between resting and maximum instead of measuring from zero. Reach for the ladder when you want the whole map and for zone 2 when you want one band to hold steady on a device. Formula zones are misleading for anyone taking heart-rate-affecting medication such as beta blockers, and a measured maximum from a supervised test always beats the estimate.

Everything runs in your browser

Nothing entered on this site is transmitted anywhere. Each formula is a small pure function that loads with the page and runs locally — there is no account, no submit step, and nothing stored on a server. The worked examples above are computed by those same functions at build time, so the prose and the code cannot drift apart: if a sentence disagreed with the engine, the site would fail to build instead of publishing a wrong number.

Guides, and how to check our work

The longer explanations live in the guides, which carry the reasoning that will not fit beside a result. Calorie needs explained walks the whole BMR → activity factor → target chain and dwells on the multiplier as its weakest link; what BMI does and does not measure separates the questions the ratio can answer from the ones it cannot; and how due dates are calculated explains Naegele’s rule and why gestational age is counted from the last menstrual period rather than from conception — a due date is an estimate that clinicians routinely revise, and no calculator replaces prenatal care.

For the formula-by-formula provenance, the reported error bands, the input ranges the engine accepts and why it rejects everything outside them, and how the implementations are tested against hand-computed reference values, see the methodology page. If a result ever looks wrong, the contact page explains what to send.

Not medical advice: every figure on this page is a general educational estimate produced by a published population formula. It cannot account for your individual health context and must not be used to diagnose or treat any condition — consult a qualified healthcare professional before changing diet, training, or medication. Data is processed locally in your browser and never transmitted. See the methodology page.