BMR calculator

The three BMR formulas compared

All three equations estimate the same thing in different ways. Mifflin-St Jeor (1990) and the revised Harris-Benedict (1984) use sex, age, height and weight. Katch-McArdle uses only lean body mass, which is why it needs your body fat percentage. Here’s what each gives for four example people, in calories a day:

The formulas usually land within 100 to 200 calories of each other, and that gap is smaller than the error margin of any one of them. In a 2005 systematic review, Mifflin-St Jeor predicted resting energy within 10% of the measured value for more people than the other common equations, which is why it’s the default here.

What sets your BMR

• Lean mass. Muscle and organs use far more energy than fat, so two people of the same weight can have quite different BMRs.
• Size. A taller, heavier body has more tissue to keep running.
• Sex. Men tend to carry more lean mass at the same size, so the equations add about 166 calories for men compared with women.
• Age. BMR drifts down slowly with age, mostly through losing muscle.
• Everything else. Genes, thyroid hormones, some medicines, illness and pregnancy all shift metabolism in ways no equation can see. That’s why every estimate here comes with a range.

BMR by age

For the same body, Mifflin-St Jeor takes 5 calories a day off for every year of age. The table shows a woman who is 5 ft 4 in and 150 lb and a man who is 5 ft 9 in and 190 lb, with only the age changing:

A 2021 study of more than 6,400 people measured with doubly labeled water found metabolism, adjusted for body size, stays roughly level from 20 to 60 and only then declines slowly, by about 0.7% a year. So if your needs seem to fall in your 40s, less muscle and less movement are the more likely causes, and both respond to exercise.

From BMR to daily calories

You burn more than your BMR every day: digesting food uses about a tenth of what you eat, and moving uses the rest. Multiplying BMR by an activity factor, from 1.2 for a desk job to 1.9 for hard daily training, gives your total daily energy expenditure, the calories that keep your weight steady. The table under the calculator shows every level; the TDEE calculator adds goal calories, and the calorie deficit calculator turns a weight goal into a daily target and a date.

Sources

• Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
• Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. American Journal of Clinical Nutrition, 1984.
• Pontzer H et al. Daily energy expenditure through the human life course. Science, 2021.

An estimate for healthy adults, not medical advice. If you think your metabolism is unusually slow or fast, a doctor can check for causes such as thyroid problems.