BMR calculator

What is BMR?

BMR is your basal metabolic rate: the energy your body uses in a day at complete rest. It keeps your heart, lungs, brain, and other organs going. In this example, it is 1,795 kcal a day. That is about 75 kcal an hour, or 1.2 kcal a minute.

Term	What it means
BMR	Basal metabolic rate. What you burn at complete rest, measured when you wake, 12 or more hours after a meal6.
RMR	Resting metabolic rate. What you burn at rest on a normal day, a few hours after a meal. It is about 10% higher than BMR6.

Mifflin–St Jeor and Harris–Benedict come from measurements at rest, so strictly they estimate RMR^1,3. Most calculators call the result BMR, and this page does too. For most people, your BMR is the largest part of what you burn in a day, but not all of it. Movement adds the rest, as the TDEE calculator shows.

How to calculate your BMR with the Mifflin–St Jeor equation

Mifflin–St Jeor is the formula the Fit Shapes app uses¹. It needs your weight in kilograms, your height in centimeters, your age, and your sex.

	Worked out	kcal
Weight	10 × 83.9 kg	839
Height	6.25 × 177.8 cm	1,111
Age	−5 × 32	−160
Male		+5
BMR		1,795

For a man, the formula adds 5 kcal. For a woman, it takes away 161. To use pounds and inches, multiply pounds by 0.4536 and inches by 2.54 first.

Harris–Benedict and Katch–McArdle

Harris–Benedict is an older formula, from 1918, revised in 1984³. Many calculators still use it. It has a different number for each term, and different numbers for men and women:

	Worked out	kcal
Start	88.362	88
Weight	13.397 × 83.9 kg	1,124
Height	4.799 × 177.8 cm	853
Age	−5.677 × 32	−181
BMR		1,884

Katch–McArdle starts from your lean mass: everything in your body that is not fat. It needs your body fat percentage, and it uses neither your age nor your sex. The formula is 370 + 21.6 × lean mass in kilograms, and lean mass is your weight × (1 − body fat). Cunningham published it in 1991⁴, and most calculators call it Katch–McArdle. An older formula of Cunningham’s, from 1980, is 500 + 22 × lean mass⁵.

Add your body fat in the sentence at the top, and this line shows your Katch–McArdle number.

Which BMR formula is most accurate?

In a 2005 review of the formulas that dietitians use most, Mifflin–St Jeor came within 10% of measured resting burn more often than the others, and its errors had the narrowest range². The same review says that older adults and ethnic minorities in the US were underrepresented in the studies behind these formulas.

In this example, 10% is about 180 kcal a day, either way. The review did not test Katch–McArdle. It needs a body fat number, and that number is often an estimate too, so its error moves to a different place.

What is a normal BMR?

In the study behind Mifflin–St Jeor, measured resting burn averaged about 1,350 kcal a day for 247 women and about 1,780 for 251 men¹. The range was wide: from 927 to 2,216 kcal for the women, and from 1,030 to 2,849 for the men.

So there is no one normal number. The formula moves with each answer. In Mifflin–St Jeor:

• Each kilogram of weight adds 10 kcal a day.
• Each centimeter of height adds 6.25 kcal a day.
• Each year of age takes away 5 kcal a day.
• At the same size and age, a woman’s BMR is 166 kcal a day lower than a man’s.

Should you eat your BMR to lose weight?

No. Your BMR is the floor of what you burn, not a target. Even if you rarely work out, you burn about 2,154 kcal a day. If you ate only your BMR, you would eat 359 kcal a day under that.

Fit Shapes never sets a calorie target under the larger of 1,200 kcal and 60% of what you burn. In this example, at the lowest activity level, that is 1,293 kcal a day. To find a target for your goal, use the TDEE calculator.

Does BMR drop with age and with weight loss?

In the formula, yes. Each year takes 5 kcal a day off Mifflin–St Jeor’s answer, so ten years take 50. Each kilogram you lose takes 10 kcal a day off. So the same calorie target makes a smaller gap as you lose weight.

Can you raise your BMR?

A little, mostly by adding muscle. At rest, a kilogram of muscle burns about 13 kcal a day, and a kilogram of fat about 4.5⁷. So 2 kg more muscle adds only about 26 kcal a day.

Your organs burn much more for their size. The liver burns about 200 kcal per kilogram a day, the brain about 240, and the heart and kidneys about 440 each⁷.

Your BMR in Fit Shapes

Fit Shapes uses Mifflin–St Jeor for the resting part of what you burn. On the Today screen, the burn ring lays your resting burn down through the day, as the clock at the top of this page does, and adds the active energy that Apple Health or Health Connect measures.

A formula is still a guess. Adaptive Targets measures what you really burn from your food log and your weight trend, and suggests new targets each week.

Questions

Do you burn calories while you sleep?

Yes. Your BMR is the energy your body uses at rest, and it does not stop at night. The clock on this page spreads it evenly over the day, as the Fit Shapes app does.

Can I use the body fat number from my smart scale?

Yes. Type it in the body fat box after you choose Katch–McArdle. A scale estimates body fat, so the answer is an estimate too.

Can a teenager use this calculator?

No. It is for adults 18 and older. For a younger person, ask a doctor or a registered dietitian.

Does this calculator save my answers?

Only in this browser tab, and only until you close it. The math runs in your browser, and nothing you type leaves it. The tab keeps your answers so that the other calculators can use them.

Sources

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. doi.org/10.1093/ajcn/51.2.241
2. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775–789. doi.org/10.1016/j.jada.2005.02.005
3. Harris JA, Benedict FG. A biometric study of human basal metabolism. Proc Natl Acad Sci USA. 1918;4(12):370–373. Revised in: Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr. 1984;40(1):168–182. doi.org/10.1073/pnas.4.12.370
4. Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr. 1991;54(6):963–969. doi.org/10.1093/ajcn/54.6.963
5. Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr. 1980;33(11):2372–2374. doi.org/10.1093/ajcn/33.11.2372
6. National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Energy. Washington, DC: The National Academies Press; 2023. Chapter 4, Table 4-1. doi.org/10.17226/26818
7. Wang Z, Ying Z, Bosy-Westphal A, et al. Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure. Am J Clin Nutr. 2010;92(6):1369–1377. Citing Elia M, 1992. doi.org/10.3945/ajcn.2010.29885