What is BMR? Basal metabolic rate explained

Your body burns energy even when you do nothing. Organs need fuel, cells turn over, and your core temperature stays close to 37 °C. BMR is the price of that upkeep. It is the foundation of every calorie estimate: calculators predict BMR first, then multiply up for activity.

BMR vs RMR

The two terms are often swapped. They are close, not identical.

• BMR: Basal metabolic rate, measured under strict conditions. You lie down, awake, in a comfortable room, after a night’s sleep and an overnight fast, with no hard exercise the day before.
• RMR: Resting metabolic rate, measured with looser rules, usually after a short rest rather than on waking. It tends to read a little higher than BMR, often by less than 10%.

The Mifflin-St Jeor equation was built from resting measurements, so strictly it predicts RMR, even when a calculator labels the result BMR. For planning food, the difference is small next to the error in any estimate.

What drives BMR

Most of your BMR comes from a few small organs. The liver, brain, heart and kidneys make up only about 5–6% of body weight but account for roughly 60% of resting energy use. Muscle burns far less per kilogram, but there is a lot of it.

The figures are from Elia (1992), still the standard values in metabolism research. They explain the main drivers:

• Lean mass: The strongest single factor. In a 2005 study of 150 adults by Johnstone and colleagues, fat-free mass explained about 63% of the differences in BMR between people.
• Body size: Bigger bodies have more tissue to maintain, so taller and heavier people burn more at rest.
• Age: BMR tends to fall with age, mostly because lean mass falls. A large 2021 analysis led by Herman Pontzer found that, adjusted for body size and composition, metabolism held steady from about 20 to 60 and then declined by about 0.7% a year.
• Sex: Men usually have a higher BMR, mainly because they carry more lean mass at the same weight. In the Johnstone study, sex added little once fat-free mass was accounted for.
• Everything else: Thyroid hormones, fever, illness and some medications move BMR. About a quarter of the variation in the Johnstone study had no clear cause.

The equations, by name

Outside a lab, BMR is predicted from body measurements. Three equations do most of the work.

• Mifflin-St Jeor (1990): Uses weight, height, age and sex. A 2005 review for the American Dietetic Association by Frankenfield and colleagues found it predicted resting energy use within 10% of measured values in more people than the other equations tested. It is the default in most calculators.
• Harris-Benedict: The original equation from 1919, revised by Roza and Shizgal in 1984. It tends to read a little higher than Mifflin-St Jeor.
• Katch-McArdle: Uses lean body mass, so it needs a body-fat estimate. It ignores age and sex, which suits very muscular people, and it is only as good as the body-fat number you give it.

A worked example

Take a 40-year-old man who weighs 85 kg and stands 178 cm tall.

• Mifflin-St Jeor: (10 × 85) + (6.25 × 178) − (5 × 40) + 5 = 1,767.5, so about 1,768 kcal.
• Harris-Benedict, revised: 88.362 + (13.397 × 85) + (4.799 × 178) − (5.677 × 40) = about 1,854 kcal.
• Katch-McArdle at 22% body fat: lean mass is 85 × 0.78 = 66.3 kg, and 370 + (21.6 × 66.3) = about 1,802 kcal.

The three answers sit within about 90 kcal of each other, a spread of roughly 5%. Now change one input. Run Mifflin-St Jeor for a woman with the same weight, height and age and the result drops by 166 kcal to about 1,602. Make him 60 instead of 40 and it drops by 100 kcal to about 1,668. Each formula is a population average, and your true number can sit 10% or more away from all of them. The BMR calculator does the arithmetic for your own inputs.

Can you boost your metabolism?

Mostly, no. The claims fall into three groups.

• Foods and supplements: Caffeine, green tea and chili raise energy use slightly for a few hours. The effect is small, and it tends to be smaller in people who consume them every day.
• Meal timing: Six small meals burn no more in digestion than three larger ones. The cost tracks how much you eat, not how often.
• Muscle: Muscle does burn more than fat at rest, but only about 13 kcal per kilogram per day against about 4.5 for fat. Gaining 2 kg of muscle adds roughly 26 kcal a day, about four almonds.

Muscle is worth building for strength, not for a faster BMR. The real levers on daily energy sit outside BMR: how much you train, how much you eat, and how much you move between meals, which is NEAT.

Common mistakes

• Eating to BMR: BMR is not a diet target. It leaves out all activity and digestion, so eating at it is a large deficit for almost everyone.
• Blaming metabolism first: In a 1992 study by Lichtman and colleagues, people who said they could not lose weight on low intakes had normal measured metabolism. They had underreported what they ate by nearly half.

How FORM uses it

FORM starts your calorie target from Mifflin-St Jeor, using the age, sex, height and weight you enter, then multiplies by an activity factor for your day. Every target is editable, and your intake shows up against it in the macro tracker. For how BMR fits into the rest of your daily burn, read what TDEE is.

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