BMR Calculator

Basal metabolic rate is the energy your body spends doing nothing at all: pumping blood, breathing, holding a temperature, running the brain and kidneys and liver. It is the largest single component of most people's daily expenditure, usually somewhere between 60% and 70% of it, and it is the floor everything else is added to.

The equation this page uses

The estimate comes from the Mifflin-St Jeor equation, which predicts resting rate from weight, height, age and sex:

Men:   10 x weight(kg) + 6.25 x height(cm) - (5 x age) + 5
Women: 10 x weight(kg) + 6.25 x height(cm) - (5 x age) - 161

A 30-year-old man of 80 kg and 180 cm gives 800 + 1,125 - 150 + 5 = 1,780 kcal a day. The calculator also breaks that into 74.2 kcal an hour, and shows 2,136 kcal as the sedentary-day figure — the resting rate multiplied by 1.2, which is roughly what a desk-bound day costs.

Why Mifflin-St Jeor rather than Harris-Benedict

The Harris-Benedict equation dates from 1919 and was fitted to a small sample of adults whose body composition differs from the modern population it is still applied to. Frankenfield and colleagues reviewed the predictive equations against measured resting expenditure in a 2005 systematic review in the *Journal of the American Dietetic Association*, and Mifflin-St Jeor came out as the most reliable of the widely used formulas in non-obese adults. It is the default in most clinical nutrition software for that reason.

Reliable here means less wrong on average. For an individual the prediction still typically lands within about 10% of the measured value, which on 1,780 kcal is a spread of roughly 180 kcal in either direction. The equation cannot see your body composition: it infers from height, weight, age and sex, so two people of identical build and very different muscle mass receive identical answers, and the more muscular of the two burns more than predicted.

This number is not a calorie target

The most common misreading of a basal rate is to treat it as an intake goal. Eating 1,780 kcal because that is your resting expenditure means eating well under what a day of walking, working and standing costs — the deficit is unintentional and often large. A resting rate becomes an intake figure only after multiplying by an activity factor, which is what the calorie calculator does.

There is also a naming muddle worth knowing about. True basal rate is measured after an overnight fast, at rest, in a thermally neutral room, before the subject has got up. Resting metabolic rate is measured under looser conditions and comes out perhaps 10% higher. Predictive equations were validated against resting measurements, so what this page gives you is closer to RMR than to strict BMR, whatever the label says.

Where the estimate breaks down

The equation was fitted to adults and should not be applied to children. It does not hold in pregnancy, and it has no way to account for untreated thyroid disease, significant illness, or the metabolic adaptation that follows a long period of restriction — all of which move resting expenditure away from what height and weight predict. At the extremes of body size the prediction degrades further, since very heavy adults carry a lower proportion of metabolically active tissue than the formula assumes.

If you want a rough sense of how your build compares with population averages, the BMI calculator uses the same two measurements for a different purpose. The calculation here happens entirely in your browser, your figures are not transmitted, and your own browser may keep them so the form is still filled in when you return. No clinician has reviewed this page.