BMR Calculator — Basal Metabolic Rate

Your basal metabolic rate is the energy your body burns doing nothing at all — the cost of staying alive for 24 hours. It is the largest single piece of what you spend in a day, and the standard way to estimate it is the Mifflin-St Jeor equation, which needs only your sex, age, weight and height. Enter them below.

The equation, and the conversion people get wrong

Mifflin-St Jeor is defined in kilograms and centimeters, not pounds and inches. Convert first, then calculate — doing it the other way around is the single most common mistake:

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
• Women: the same line, but − 161 instead of + 5

Take a 35-year-old man at 180 lb and 5 ft 10 in. That is 180 ÷ 2.205 = 81.6 kg, and 70 inches × 2.54 = 177.8 cm. So:

10 × 81.6 + 6.25 × 177.8 − 5 × 35 + 5 = 816 + 1,111 − 175 + 5 = 1,757 kcal/day.

Now a 35-year-old woman at 150 lb and 5 ft 5 in — 68.0 kg and 165.1 cm:

10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031 − 175 − 161 = 1,375 kcal/day.

If the arithmetic is where you slip, our pounds to kg and feet and inches to meters converters do that step alone.

How wrong the estimate can be

This matters more than the number itself. Predictive equations are not measurements — the measurement is indirect calorimetry, done in a lab with a metabolic cart. When the Academy of Nutrition and Dietetics reviewed the competing equations, Mifflin-St Jeor came out best, and “best” looked like this: it lands within 10% of the measured value in about 82% of non-obese adults and 70% of adults with obesity. In people with obesity the misses skew low, by as much as 21%.

Read that the other way and it is sobering. Roughly one person in five is off by more than a tenth. On a 1,757 kcal estimate, a 10% error is 176 kcal a day in either direction — enough to turn a planned deficit into maintenance, or the reverse. Treat the output as the middle of a band, not a target you hit.

Your BMR is not how much you can eat

The most expensive misreading of this page would be to eat 1,757 kcal because that is what the calculator said. Basal rate is what you would burn lying perfectly still all day. Total daily energy expenditure adds everything else: walking to the car, standing, fidgeting, working out, and the 10% or so your body spends digesting food.

The usual bridge is an activity multiplier applied to BMR:

Activity level	Factor	Total for a 1,757 kcal BMR
Sedentary — little or no exercise	× 1.2	2,108 kcal/day
Light — 1 to 3 days/week	× 1.375	2,416 kcal/day
Moderate — 3 to 5 days/week	× 1.55	2,723 kcal/day
Intense — 6 to 7 days/week	× 1.725	3,031 kcal/day
Very intense — daily hard training or physical work	× 1.9	3,338 kcal/day

The gap between the first and last row is 1,230 kcal, which is why guessing your activity level generously is a bigger source of error than the equation itself. The difference between BMR and TDEE page unpacks that step.

What actually sets your basal rate

Not willpower, and not much that you can buy. The 2023 Dietary Reference Intakes are blunt about the main driver: fat-free mass accounts for 60 to 80 percent of the differences in resting energy expenditure between individuals, while body weight on its own explains only about half. Once body composition is accounted for, sex has little independent effect — the +5 and −161 constants in the equation are standing in for an average difference in lean mass, not for something intrinsic about being male or female.

That is also why a formula built on weight, height, age and sex cannot see the thing that matters most. It has no idea whether your 180 lb carries a lot of muscle or a little. If your result feels wrong, that is usually the reason — and it is worth reading before concluding you have a slow metabolism.

Pick the version that fits your case

The equation is the same; the framing changes:

• BMR calculator for women and for men — the same math with the sex constant already applied.
• Mifflin-St Jeor calculator and Harris-Benedict calculator — the two equations side by side, with the coefficients as published.
• BMR calculator for seniors and BMR for teens — where the adult equation starts to strain.
• How to speed up metabolism — what moves the number, and what only claims to.

This content is informational and does not replace evaluation by a physician or registered dietitian.