BMR calculator: basal metabolic rate, 3 formulas

What is BMR

BMR, basal metabolic rate, is the energy your body uses at complete rest: breathing, circulation, keeping warm, running the brain and organs. It is what you would burn lying still for a whole day, and for most people it is 60 to 70 percent of everything they burn.

BMR is not how much to eat. Your daily needs are BMR plus digestion, movement and exercise, which is what the TDEE calculator adds up.

The BMR formulas

None of these measures you. Each predicts the average BMR of people like you, from studies that measured it directly. This calculator shows all three (Katch-McArdle once you enter a body fat percentage). The gap between them shows how much the choice of equation matters; your own error can be larger than that gap, because all of them can miss the same person in the same direction.

All three take metric units. In US units, weight in kg = pounds ÷ 2.205 and height in cm = inches × 2.54.

Mifflin-St Jeor (1990) is the equation the Academy of Nutrition and Dietetics recommends. A 2005 systematic review found it predicted measured resting expenditure to within 10 percent for about 70 to 80 percent of people, the best of the common equations.

• Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
• Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

For example, a 35-year-old woman who is 5 ft 5 in (165 cm) and 160 lb (72.6 kg): 726 + 1,032 − 175 − 161 ≈ 1,420 calories a day.

Harris-Benedict dates from 1919. This calculator uses the 1984 revision by Roza and Shizgal. It tends to come out a little higher than measured in modern populations.

• Men: 88.362 + 13.397 × weight (kg) + 4.799 × height (cm) − 5.677 × age (years)
• Women: 447.593 + 9.247 × weight (kg) + 3.098 × height (cm) − 4.330 × age (years)

Katch-McArdle ignores age and sex and works from lean body mass alone: 370 + 21.6 × lean mass (kg). It suits people who are unusually muscular or unusually lean, whom the other two misjudge. But it is only as good as your body fat figure. An error of 5 percentage points in body fat moves the result by about 90 calories for an 80 kg person. The body fat calculator estimates it from a tape measure.

BMR vs RMR

Strictly, BMR is measured after a night's sleep, fasted, in a warm, quiet room. RMR, resting metabolic rate, is measured under looser conditions and usually runs up to about 10 percent higher. The equations differ here too: Harris-Benedict was built from basal measurements, Mifflin-St Jeor from resting ones. For setting how much to eat, the difference is smaller than the error in either equation.

What raises or lowers BMR

• Size and lean mass. A larger body burns more. Muscle burns about 13 calories per kilogram a day at rest and fat about 4.5, so muscle helps, but gaining 2 kg of it raises BMR by only about 25 calories a day.
• Sex. Men's BMR is higher on average, mostly because they carry more lean mass at the same weight.
• Age. Less than people think. A 2021 study in Science measured energy expenditure in more than 6,000 people. Once adjusted for body size and composition, it was stable from about 20 to 60 and fell about 0.7 percent a year after that. The equations still subtract calories for age, because they cannot see the lean mass people tend to lose as they get older.
• Dieting. Losing weight lowers BMR, and a long, hard diet lowers it a little more than the weight loss alone explains.
• Thyroid function and some medications. An underactive thyroid lowers BMR. If your weight changes without an explanation, that is a question for a doctor, not a calculator.

Is it bad to eat below your BMR?

There is no physiological switch at BMR: the body's adaptation to dieting scales with the size of the deficit, not with crossing a particular line. What matters is the deficit relative to your TDEE, whether you get enough protein and micronutrients, and whether you can keep it up. That said, eating below BMR usually means a large deficit. If you have a history of disordered eating, set your intake with a clinician. The calorie deficit calculator flags intakes that are too low to follow without supervision.