BMR Calculator

How BMR is calculated

Basal metabolic rate is the energy your body burns at complete rest. This calculator supports the three most-used equations: Mifflin-St Jeor (the modern default), the revised Harris-Benedict, and Katch-McArdle for people who know their body fat percentage. Imperial units are converted to metric automatically.

Formulas

Mifflin-St Jeor (men): 10 × kg + 6.25 × cm − 5 × age + 5

Mifflin-St Jeor (women): 10 × kg + 6.25 × cm − 5 × age − 161

Harris-Benedict (men): 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age

Harris-Benedict (women): 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age

Katch-McArdle: 370 + 21.6 × lean mass (kg)

Worked example

A 30-year-old woman, 70 kg, 170 cm, with Mifflin-St Jeor: 10 × 70 + 6.25 × 170 − 5 × 30 − 161 = 700 + 1,062.5 − 150 − 161 = ≈ 1,452 kcal/day. That is roughly 10,160 kcal per week just to keep the body running at rest.

Which formula should you use?

Mifflin-St Jeor is the modern default and the most accurate for the general population, so use it unless you have a reason not to. Harris-Benedict is the older equation, kept here for comparison and for anyone whose plan or coach references it. Katch-McArdle can be more accurate for lean, muscular people because it works from lean body mass instead of total weight — but it only helps if you have a reliable body-fat measurement, since a rough guess feeds a rough result.

BMR, RMR, and your real burn

BMR is measured under strict resting conditions; resting metabolic rate (RMR) is a slightly looser, higher figure you will often see used interchangeably. Either way, this is only the energy to keep you alive at rest — it excludes walking, exercise, and even digesting food. Your actual daily burn is BMR multiplied by an activity factor, which is exactly what the calorie calculator does next.

Harris-Benedict: the 1919 original and the 1984 revision

“The Harris-Benedict equation” refers to two different sets of coefficients, and calculators rarely say which one they ran. The original was published in 1919; Roza and Shizgal revised it in 1984 against better data. This calculator uses the 1984 revision, which is the version cited in modern nutrition texts.

1919 original (men): 66.473 + 13.7516 × kg + 5.0033 × cm − 6.755 × age

1919 original (women): 655.0955 + 9.5634 × kg + 1.8496 × cm − 4.6756 × age

The two agree closely for average builds — under 10 kcal apart for the worked example above — and diverge for people who are taller, heavier or older. If a coach's spreadsheet disagrees with this page by a small margin, a 1919/1984 mismatch is the usual reason.

How far apart the formulas actually land

All four equations, run on three different people, so you can see where the choice matters and where it does not:

For a person of average build the choice is worth around 50 kcal — noise, next to the error in food tracking. It matters at the edges: in the third profile the Harris-Benedict figures sit nearly 200 kcal above Katch-McArdle, because they scale with total weight while Katch-McArdle scales with lean mass and ignores the fat entirely. That is precisely the case where knowing your body fat percentage buys you a better answer.

Working from body fat percentage or lean mass

Lean body mass = weight × (1 − body fat % ÷ 100)

Katch-McArdle is the only equation here that takes lean mass rather than sex, height and age. At 80 kg and 20% fat the lean mass is 64 kg, giving 370 + 21.6 × 64 = 1,752 kcal. Two people of identical weight and 10 points of body fat between them get answers about 170 kcal apart — a difference the weight-based formulas cannot see at all.

The catch is input quality. A caliper or tape estimate carries several points of error, and that error flows straight into the result. Use Katch-McArdle when the body fat figure came from DEXA or a consistent method you trust; otherwise Mifflin-St Jeor from plain weight is the safer bet. The body fat calculator estimates the percentage from tape measurements.

How accurate is any of this?

Mifflin-St Jeor lands within 10% of measured resting energy expenditure for roughly four people in five — which also means one in five sits further out, and a 10% miss on 1,800 kcal is 180 kcal a day. The equations are least reliable at the extremes of body composition, in the very old, and in people who have dieted for long periods, where measured rates commonly run below prediction.

Treat the number as a starting point with a margin, not a measurement. Hold intake steady for two to three weeks, track weight, and adjust from what actually happened — the observed trend beats any equation.

BMR is only the baseline — to turn it into a daily calorie target that accounts for activity and your goal, use the calorie calculator.