BMR Calculator

What is BMR?

Basal Metabolic Rate is the number of calories your body needs just to stay alive at complete rest — breathing, circulating blood, repairing cells, regulating body temperature. It typically makes up 60–75% of your total daily calorie burn, before any activity is added on top, which is why it's the starting point for most calorie and weight-management calculations.

Organs do most of that work. Your brain, liver, heart and kidneys account for a large share of resting energy use despite being a small part of body weight, and skeletal muscle adds more as it grows. That's why two people of the same weight can have different BMRs: the one carrying more muscle and less fat usually burns more at rest.

BMR vs. RMR vs. TDEE

These three terms get used interchangeably, but they measure different things:

• BMR (basal metabolic rate) is measured under strict lab conditions: after a night's sleep, a 12-hour fast, lying still in a warm, quiet room.
• RMR (resting metabolic rate) is measured under looser conditions, so it runs slightly higher than BMR. The Mifflin-St Jeor paper actually predicts resting energy expenditure, and in everyday use the two are treated as the same number.
• TDEE (total daily energy expenditure) is everything you burn in a day: BMR plus walking, exercise, fidgeting and the energy used to digest food. Calculators estimate it by multiplying BMR by an activity factor between 1.2 (sedentary) and 1.9 (very active).

For eating targets you want TDEE, not BMR. The Calorie Calculator applies those activity factors to the same Mifflin-St Jeor estimate and shows maintenance, loss and gain targets, and the macro calculator splits that total into protein, carbs and fat.

How BMR is calculated

Both formulas on this page use weight, height, age, and sex — the Mifflin-St Jeor equation is 10 × weight(kg) + 6.25 × height(cm) − 5 × age, +5 for men or −161 for women. Harris-Benedict uses a similar but separately-weighted formula. Neither accounts for body composition directly, which is why two people with the same height, weight, and age but very different muscle mass can have noticeably different actual BMRs.

Worked example: a 30-year-old man who weighs 154 lb (69.9 kg) and stands 5 ft 9 in (175.3 cm) gets 10 × 69.9 + 6.25 × 175.3 − 5 × 30 + 5 ≈ 1,649 calories a day. A woman with the same stats gets 166 fewer, about 1,483. If you work in pounds and inches, the calculator converts for you; to check a conversion by hand, see kg to lbs.

Average BMR by age and sex

The table below runs the Mifflin-St Jeor equation for two example bodies at different ages: a man at 5 ft 10 in and 180 lb, and a woman at 5 ft4 in and 150 lb. These are illustrative reference figures, not measured population averages. They show how the estimate moves with age when nothing else changes.

Each decade lowers the estimate by 50 calories a day, because the equation's only age term is −5 per year. Weight moves it more: every extra 10 kg (22 lb) adds 100 calories a day.

Mifflin-St Jeor vs. Harris-Benedict

Harris-Benedict dates to 1918 (revised in 1984 by Roza and Shizgal); Mifflin-St Jeor, published in 1990, was developed using more modern measurement techniques and is generally considered more accurate for today's population, which is why most fitness apps and dietitians default to it. The two formulas usually land within a few percent of each other, but Mifflin-St Jeor is the safer default if you only want one number.

A third option, the Katch-McArdle formula (370 + 21.6 × lean body mass in kg), skips sex, height and age and works from fat-free mass instead. It can suit very muscular people whose weight overstates their fat, but it's only as good as the body-fat estimate behind it. If you want to try it, get a tape-measure estimate from the body fat calculator first.

How accurate is a BMR calculator?

Both formulas are typically accurate to within about 10% for most people, but they're population averages, not individual measurements — the only way to know your exact BMR is indirect calorimetry, a clinical test that measures actual oxygen consumption. Use this calculator's output as a solid starting estimate, then adjust based on real results if you're tracking weight over time.

A practical check: eat at your estimated maintenance for two to three weeks and weigh yourself several times a week. If the weekly average drifts down, your real needs are higher than the estimate; if it creeps up, they're lower. People with thyroid conditions, those who are pregnant, and anyone under 18 should rely on advice from a clinician rather than a population equation.

Sources

The formulas on this page come from the published research below. Last updated October 11, 2026. This calculator gives estimates for general information. It isn't medical advice, so check anything important with a qualified professional.

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247.Used for: Mifflin-St Jeor BMR equation
2. Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168–182.Used for: Revised Harris-Benedict equation (comparison figure)
3. Harris JA, Benedict FG. A Biometric Study of Human Basal Metabolism. Proc Natl Acad Sci USA. 1918;4(12):370–373.Used for: Original Harris-Benedict equation