BMR Calculator

What BMR actually measures

Basal metabolic rate is the energy cost of keeping you alive while you do nothing: breathing, circulating blood, maintaining body temperature, running the brain and organs. For most adults it accounts for roughly 60–75% of all calories burned in a day, which is why it is the starting point for every calorie calculation — but it is not the whole story, because it excludes every movement you make.

The figure is defined under strict conditions — lying down, awake, rested, in a comfortable temperature, having fasted — so no one ever spends a day exactly at BMR. Treat the result as a baseline: your real daily burn is always higher, by an amount that depends on how much you move. Our calorie calculator adds that activity on top to give total daily expenditure.

The Mifflin-St Jeor equation

Published in 1990, the Mifflin-St Jeor equation is now the standard estimator: for men, BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age + 5; for women, the same with −161 in place of +5. A 40-year-old man who is 180 cm and 85 kg comes out at about 1,780 kcal a day. It replaced the older Harris-Benedict equation because it matched measured resting expenditure more closely in validation studies.

The formula uses only four inputs, so it necessarily averages over everything it cannot see. Muscle burns more at rest than fat, so two people of identical height, weight, age and sex can have genuinely different BMRs if one is muscular and the other is not. Illness, fever, pregnancy, thyroid disorders and some medicines also shift resting burn well outside what any four-input formula can predict.

BMR, RMR and TDEE — keeping them straight

You will also meet resting metabolic rate (RMR), which is measured under slightly less strict conditions than BMR and usually comes out a little higher — often by around 10%. In practice the two are used interchangeably online, and the difference matters far less than the gap between either of them and TDEE, your total daily expenditure, which adds work, exercise and everyday movement.

The practical use of BMR is as an anchor. If a diet plan asks you to eat well below your BMR for weeks on end, that is a warning sign: sustained intakes below resting needs are difficult to maintain and can cost muscle alongside fat. Sensible weight-loss targets sit below TDEE but at or above BMR, with the gap — not the BMR figure itself — doing the work.