How these calculators work

Every number this site produces comes from a published equation. This page names each one, links to the paper it came from, and states where the result is likely to be wrong. Where a figure is an approximation rather than a measurement, this page says so instead of rounding it into confidence.

None of these tools is reviewed by a physician or a registered dietitian. What they are is transparent: the arithmetic is written out, the sources are linked, and the error is bounded. That is a narrower claim than medical authority, and it is a claim we can actually back up.

The common foundation: TDEE and maintenance calories

Seven of the eleven calculators — every calorie, macro, protein, bulking and recomposition tool — sit on top of one estimate: your total daily energy expenditure. So it is worth being precise about what that is.

Basal metabolic rate is estimated with the Mifflin-St Jeor equation, published in 1990, which takes weight, height, age and sex and returns the calories your body would burn lying still. That result is then multiplied by an activity factor to estimate a full day:

Mifflin-St Jeor replaced the earlier Harris-Benedict (1918) equation, which is still what many people mean when they say "a BMR formula". This site does not use it. It is listed in the references because you will meet it elsewhere, and knowing which equation produced a number is part of judging that number.

The Mifflin-St Jeor equation is reported to land within roughly 10% of measured resting expenditure for most adults. The activity factor is the weaker half of the estimate: it is a five-step scale, so it cannot tell a desk job from a job spent on your feet, and it asks you to pick the band that matches an average week rather than your best one. If your activity sits between two bands, the honest move is to treat the result as a range.

Note also what this equation ignores: two people of identical weight and height get an identical number regardless of how much muscle one of them has. It is a population estimate, not a measurement of you.

Converting a goal into calories

The calorie deficit and calorie surplus calculators turn a weekly rate of weight change into a daily number using the rule of thumb that 1 kg of body fat holds about 7,700 kcal. A request to lose 0.5 kg per week becomes a daily shortfall of roughly 550 kcal, subtracted from your maintenance figure.

That constant is the energy density of adipose tissue, and it is worth being honest that it is an approximation. Bodies do not surrender fat at a perfectly predictable rate, and both sides of the equation — what you eat, what you burn — are themselves estimates. The arithmetic is exact; the inputs are not.

Two guardrails apply. The deficit calculator will not return a target below 1,200 kcal, and the surplus calculator will not return one above 5,000 kcal. Those are limits built into this site to stop the tools handing out extreme numbers. They are not a claim that either figure is safe for every person, and neither is a substitute for medical advice.

Macro and protein targets

The macro calculator sets protein at 1.8 g per kg of body weight, gives fat a quarter of total calories, and fills the remainder with carbohydrate. The displayed percentages are derived from the rounded gram targets rather than the reverse, so the split you see always adds up to 100%. If a low calorie target leaves carbohydrate near zero, that is arithmetic rather than advice: protein and fat are assigned first, and carbohydrate absorbs what is left.

The protein calculator reports a range rather than a single number, because the research supports a range and one figure would imply precision that does not exist:

These are training-nutrition ranges drawn from strength-sport practice, not clinical intakes. They are calculated from total body weight, which means they will be higher than ideal for someone carrying a large proportion of body fat. Anyone with kidney disease should treat the appropriate range as a clinical decision rather than a lookup.

Body composition

The BMI calculator divides weight in kilograms by height in metres, squared, and sorts the result into categories using WHO boundaries. BMI is a screening measure, not a measure of body fat, and it cannot distinguish a muscular person from a sedentary one of the same weight and height. It covers adults only — there is no child or teen percentile logic here, and no age or sex adjustment, even though both fields are collected. Nothing it reports is a diagnosis.

The body fat calculator uses the US Navy circumference method, published by Hodgdon and Beckett in 1964, which estimates body fat from height plus tape measurements around the neck, waist and — for women — hips. It is good for tracking direction of travel and mediocre as an absolute reading: expect single-digit percentage points of error, and note that the equation was derived on a specific population. Two details worth knowing: body weight takes no part in the percentage at all, and neither does age. Weight is used only afterwards to convert the percentage into fat mass and lean mass.

That is the limit of the whole approach. Tape-measure methods are estimates; DEXA and similar techniques measure body composition directly. The lean body mass calculator takes whichever body-fat figure you supply and splits your weight withweight × (1 − body-fat %). It is exact arithmetic on an estimate, and it will faithfully reflect a bad input.

What these tools will not tell you

The body recomposition calculator is the clearest example of a deliberate limit. It tells you how many calories to eat and how much protein to aim for, then stops. It does not predict how much muscle you will gain or how much fat you will lose, because no single calorie formula can, and a number that implied otherwise would be invented. Recomposition runs at a 20% deficit — gentler than the 15% used by the macro calculator's cut setting, because losing lean mass is the main risk of getting it wrong.

It also cannot tell you that recomposition is happening. Weight may not move at all while composition changes underneath, and no calorie tool can detect that. Progress measured only on the scale will look like nothing is happening.

Every formula at a glance

The same information as above, in one table, with the limitations stated rather than implied.

If a number here is wrong

Then the site is wrong, and we would like to know. Formulas get revised, constants get corrected, and code gets refactored — this page is written to be checked against the source, so please tell us which figure you believe is incorrect and what you believe it should be. Corrections get made and the review date at the top of this page moves.