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. For most people it's the biggest single part of daily expenditure, usually somewhere between 60% and 70% of it, and everything else gets added on top.

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. That's the resting rate multiplied by 1.2, which is roughly what a desk-bound day costs.

Why Mifflin-St Jeor and not Harris-Benedict

The Harris-Benedict equation dates from 1919. It was fitted to a small sample of adults whose body composition differs from the modern population it's 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. That's why most clinical nutrition software uses it as the default.

"Reliable" here means less wrong on average. For any one person the prediction still typically lands within about 10% of the measured value, and on 1,780 kcal that's a spread of roughly 180 kcal either way. The equation can't see your body composition. It works only from height, weight, age and sex, so two people of identical build but very different muscle mass get the same answer, and the more muscular one burns more than predicted.

Don't use this number as a calorie target

The most common mistake with a basal rate is treating it as an intake goal. If you eat 1,780 kcal because that's your resting expenditure, you're eating well under what a day of walking, working and standing actually costs, and the deficit is unintentional and often large. You only get an intake figure after multiplying the resting rate by an activity factor, which is what the calorie calculator does.

The naming is a bit muddled, too. True basal rate is measured after an overnight fast, at rest, in a thermally neutral room, before the person 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 whatever the label says, the figure on this page is closer to RMR than to strict BMR.

Where the estimate breaks down

The equation was fitted to adults and shouldn't be used for children. It doesn't hold in pregnancy. It also has no way to account for untreated thyroid disease, significant illness, or the metabolic adaptation that follows a long period of restriction, and any of these can move resting expenditure away from what height and weight predict. At the extremes of body size the prediction gets worse again, because very heavy adults carry a lower proportion of metabolically active tissue than the formula assumes.

If you want a rough idea 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 and your figures aren't transmitted, though your own browser may keep them so the form is still filled in when you come back. No clinician has reviewed this page.