BMR Calculator

What BMR measures — and what it does not

BMR is measured in a laboratory under strict conditions: lying down, awake, after a night's sleep and a 12-hour fast, in a room that is neither warm nor cool. What remains is the cost of staying alive — the heart beating, the lungs breathing, the brain running, the kidneys filtering, and every cell maintaining itself. The liver, brain, heart and kidneys weigh only a few kilograms between them but account for a disproportionately large share of it.

Resting metabolic rate (RMR), the term you will also see, is measured under looser conditions and usually comes out slightly higher. Mifflin–St Jeor was in fact fitted to resting measurements. In everyday use the two terms are interchangeable, and the difference between them is small next to the error of any equation.

BMR is not what you burn in a day. Sitting upright, moving around and digesting food all cost extra — even the lowest activity factor, for a sedentary day, adds 20% — which is why the calorie calculator multiplies BMR by an activity factor before suggesting what to eat.

The three equations, and where each comes from

Mifflin–St Jeor (1990): 10 × weight in kg + 6.25 × height in cm − 5 × age, plus 5 for men or minus 161 for women. It was fitted on 498 healthy adults aged 19 to 78 — normal-weight and obese — whose energy use was measured by indirect calorimetry, and a 2005 systematic review for the American Dietetic Association, by Frankenfield and colleagues, found it the most reliable of the common equations for healthy adults.

Harris–Benedict is the original, first published in 1918. Roza and Shizgal re-derived its coefficients in 1984, and that revised version is the one used here: for men, 88.362 + 13.397 × weight + 4.799 × height − 5.677 × age; for women, 447.593 + 9.247 × weight + 3.098 × height − 4.330 × age. It usually gives a figure a few per cent above Mifflin–St Jeor.

Katch–McArdle: 370 + 21.6 × lean body mass in kg. It has no age or sex term at all, because it starts from the thing those terms stand in for. Muscle and organs burn energy at rest; fat tissue burns comparatively little. If you know your body fat, this formula can correct for a build that weight alone misreads.

Which result to trust

For most people, Mifflin–St Jeor. It is the default here and the equation dietitians most often reach for.

Katch–McArdle earns its place when your body composition is unusual — a muscular lifter whose weight overstates their fat, or someone whose weight is mostly fat and overstates their lean mass. But it is only as good as the body-fat figure you give it. A bathroom-scale reading can be several points out, and a 5-point error in body fat moves Katch–McArdle by about 85 kcal for an 80 kg person.

When the three disagree, the spread is information. For a 30-year-old man of 180 cm and 80 kg, Mifflin–St Jeor gives 1,780 kcal, Harris–Benedict 1,854 kcal and Katch–McArdle, at 20% body fat, 1,752 kcal — about a 6% spread. None of them is wrong. They are three averages fitted to different groups of people, decades apart, and you are one individual.

Why BMR changes — and what changes it less than you might think

Age: the two weight-based equations lower BMR as you get older — by 5 kcal a year in Mifflin–St Jeor — largely because lean mass tends to fall; Katch–McArdle has no age term and picks that change up through your body-fat figure instead. Strength training slows the decline, which makes it the most direct lever you have over your own BMR.

Weight: a smaller body burns less, so BMR falls as you lose weight — about 10 kcal a day for every kilogram lost, going by Mifflin–St Jeor's weight term alone. Some studies find an extra drop beyond what the smaller size explains, an effect called adaptive thermogenesis. It is real, but usually modest.

What moves it less than the marketing suggests: detox teas, 'fat-burning' supplements and so-called metabolism-boosting foods. None has been shown to raise BMR meaningfully or for long. The reliable levers are muscle and movement.

These figures are estimates for healthy adults, not medical advice. Illness, fever, pregnancy and thyroid conditions shift BMR in ways no equation can see, and weight that changes without an obvious reason is a question for a doctor rather than a calculator.