BMR Calculator — basal metabolic rate (Mifflin-St Jeor)

How it works

BMR is what you'd burn lying still all day — the cost of keeping your heart, brain, kidneys, and the rest running. The Mifflin-St Jeor equation, published in 1990 and the one clinical dietitians most often prefer, predicts it from four inputs: more weight and height raise it, age lowers it, and a sex constant (+5 for men, −161 for women) accounts for average body-composition differences. The older Harris-Benedict equation (shown here in its 1984 revision) uses different coefficients and usually lands 50–150 kcal higher; the comparison shows both so the gap is visible. Neither measures your metabolism — they estimate a population average for your numbers, and an individual can sit either side of it.

Worked example

For a 30-year-old man, 170 cm, 70 kg, Mifflin-St Jeor gives 10 × 70 + 6.25 × 170 − 5 × 30 + 5 = 700 + 1,062.5 − 150 + 5 = 1,617.5 kcal a day. The revised Harris-Benedict equation gives 88.362 + 13.397 × 70 + 4.799 × 170 − 5.677 × 30 ≈ 1,672 — about 54 kcal higher. A woman with the same numbers is 166 kcal lower on Mifflin (the −161 vs +5 constants): the sex term is the largest single difference between two otherwise-identical people.

Frequently asked questions

What is BMR?

The calories your body burns at complete rest over 24 hours — breathing, circulation, cell repair, keeping warm. It's the largest part of most people's daily energy use, typically 60–70%. It excludes anything you do: standing, walking, exercise, and even digesting food all add calories on top.

BMR vs RMR — what's the difference?

BMR is measured under strict conditions (fasted, fully rested, just after waking); resting metabolic rate (RMR) is measured under slightly looser conditions and comes out a little higher. The equations here technically predict resting energy, and the terms are used interchangeably in everyday use. The difference is small — a few percent — and doesn't change how you'd use the number.

Which equation should I use?

Mifflin-St Jeor is generally the most accurate for healthy adults — a 2005 systematic review found it within 10% of measured values for more people than Harris-Benedict or the older equations, which is why it's the default here. Harris-Benedict is shown for comparison and because many older tools still use it. Both are estimates; don't over-index on the exact figure.

How do I turn BMR into calories to maintain my weight?

Multiply BMR by an activity factor — roughly 1.2 for sedentary up to about 1.9 for very active. That product is your total daily energy expenditure (TDEE), the calories to maintain your weight. This page stops at resting energy; the TDEE calculator applies the activity factor and the calorie calculators handle deficits and surpluses.

Why does BMR fall with age?

Metabolically active tissue — especially muscle — tends to decline with age, and the equations build that average trend in (the −5 × age term for Mifflin). It's a population average: staying active and maintaining muscle blunts the drop, so two people of the same age can differ. The formula can't see your muscle mass; body-composition equations like Katch-McArdle do.

Does this work if I'm very muscular or have obesity?

Less well at the extremes. Mifflin-St Jeor was derived across normal weight to obesity, but formulas based on total weight over- or under-estimate for people with unusually high muscle or fat, because muscle burns more at rest than fat. If body composition is known, a lean-mass-based equation is more accurate; treat this as a starting estimate.

How accurate is this, and what does it exclude?

It's a statistical estimate, not a measurement — your true BMR can be 10% or more either side. It excludes all activity, the thermic effect of food, illness, medications, thyroid conditions, and pregnancy, any of which shift metabolism. For a measured value you'd need indirect calorimetry; for planning, this is a reasonable anchor.