Calorie & BMR Calculator

How the estimate is built

The calculation predicts basal metabolic rate from height, weight, age and sex, then multiplies by an activity factor to estimate total daily energy expenditure.

Mifflin-St Jeor is the usual equation for the first step, being the best validated of the common options in modern populations. Harris-Benedict and Katch-McArdle are alternatives, the last using lean body mass instead of total weight.

The activity multiplier runs from 1.2 for sedentary to 1.9 for extremely active, and it is the input people get most wrong. Descriptions like moderately active are vague, and the bias is consistently towards overestimating.

How wrong it can be

Mifflin-St Jeor predicts measured resting metabolic rate within 10% for around 70% of people, which leaves three in ten further out.

On a maintenance estimate of 2,200 calories, 10% is 220 calories a day. Someone eating at an estimate that is 10% high will not lose weight on a plan that assumed a 250 calorie deficit, and will conclude the plan does not work.

The activity multiplier adds its own error on top. Choosing 1.55 when 1.375 is accurate adds another 12%, and the two errors can compound in the same direction.

Correcting against reality

The estimate is a starting hypothesis. The correction comes from what actually happens: eat at the estimate for two to three weeks, weigh daily, average by week, and see whether weight moved.

Stable weight means the estimate was maintenance. A loss of half a kilogram a week means intake was roughly 500 calories a day below it.

This requires honest tracking, which is where it usually fails. Under-reporting of intake is well documented and substantial, commonly 20% or more, and it happens to people who believe they are being accurate. Weighing food rather than estimating removes most of it.

Setting a deficit or surplus

For weight loss, 0.5% to 1% of body weight per week is a commonly recommended rate, achieved through a deficit of roughly 15% to 25% below maintenance.

For weight gain, a surplus of 10% to 20% supports muscle gain without excessive fat gain in someone training. Larger surpluses add fat faster without adding muscle faster.

In both directions, the rate should be adjusted against what actually happens rather than defended because the arithmetic said so. Expenditure changes as weight changes, which means a deficit that worked in month one will not in month four without recalculation.

What the number does not cover

Nothing about food quality, micronutrients, fibre or protein adequacy. A calorie target met entirely from confectionery is met.

Nothing about individual variation in appetite, satiety or how different foods affect them. Two diets at the same calorie level can differ enormously in how difficult they are to sustain, and adherence predicts outcomes more reliably than any dietary composition.

Anyone with an eating disorder history, a medical condition affecting metabolism, or on medication that affects weight should work with a clinician rather than a calculator. Calorie counting is contraindicated in several situations and can be actively harmful in others.

One thing worth separating out is that the estimate and the tracking are two different sources of error, and only one of them is fixable. The equation's error is inherent and can be corrected by observing what actually happens to weight over a few weeks. The tracking error, from unweighed portions, unlogged cooking oil, and drinks that were not counted, is not corrected by anything except tracking more carefully, and it is usually the larger of the two. Anyone whose results disagree with their plan should suspect the log before suspecting the metabolism.