BMR Calculator — Three Equations and the Spread Between Them
Calculate basal metabolic rate with Mifflin-St Jeor, Harris-Benedict and Katch-McArdle at once, and see how far apart they land on your numbers.
- Section
- Health & Vitality
- Reviewed
- Your data
- Stays local
Add a body fat percentage to unlock Katch-McArdle, the only one of the three that works from lean mass and can therefore tell a muscular 80 kg from an unfit 80 kg.
Individual BMR scatters around any prediction by roughly ±10%, so 1,602–1,958 kcal is the honest range for this estimate.
The current default. Fitted in 1990 on a more representative sample and validates better in modern populations.
From 1919, revised by Roifman. Still the equation behind most older calculators, and it typically reads about 5% high.
Works from lean mass rather than total mass, so body composition changes the answer. The best choice when you have a measured body fat figure.
BMR is resting energy expenditure: what you would burn lying still, awake, fasted, in a thermoneutral room. TDEE multiplies it by an activity factor. Both are population regressions, and the activity multipliers are the cruder half — the step from 1.55 to 1.725 is a 11% jump applied to a self-assessed category.
If you have eight weeks of logged intake and weight, stop predicting. The adaptive TDEE calculator back-calculates maintenance from what your own body actually did, which is the only method that catches metabolic adaptation.
The formula
BMR = 370 + 21.6 × lean mass (kg)
Full derivation and the conditions it holds under are in Method and basis, below.
Method and basis
How the figure above is derived, and when not to act on it.
What BMR Is
Basal metabolic rate is what you would burn lying still, awake, fasted, in a room at a temperature your body does not have to work against. For most people it is 60–70% of total daily expenditure, which is why it is the number every calorie target gets built on.
Why This Page Shows Three Numbers
Because the equations disagree, and a calculator that prints one number hides that.
Mifflin-St Jeor (1990) is the current default. It was fitted on a more representative sample than its predecessor and validates better in modern populations.
Men: 10w + 6.25h − 5a + 5
Women: 10w + 6.25h − 5a − 161
Harris-Benedict (1919, Roifman revision) is the equation behind most older calculators. It typically reads about 5% high.
Katch-McArdle works from lean mass instead of total mass:
BMR = 370 + 21.6 × lean mass (kg)
That makes it the only one of the three that can tell a muscular 80 kg from an unfit 80 kg — and the only one that is sex-neutral by construction, because lean mass already carries the difference. It needs a measured body fat percentage, and when you have one it is the best choice here.
On a 30-year-old 80 kg man at 180 cm with 15% body fat, the three land at roughly 1,780, 1,854 and 1,839 kcal. A 4% spread, from three equations that are all considered valid.
The Activity Multiplier Is the Weaker Half
TDEE multiplies BMR by an activity factor: 1.2 for sedentary up to 1.9 for very active. Those steps are large — moving from 1.55 to 1.725 is an 11% jump — and the category is self-assessed. Most of the error in a daily calorie target comes from this step, not from the BMR equation.
Individual Scatter Is ±10%
Every equation here is a population regression. Real BMR varies around the prediction by roughly 10% in either direction, and more at the extremes of body composition. For an 1,800 kcal estimate that is a 1,620–1,980 range.
That is not a reason to ignore the number. It is a reason to treat it as a starting point and then measure: with eight weeks of logged intake and weight, the adaptive TDEE calculator back-calculates maintenance from what your own body actually did. It is the only method here that catches metabolic adaptation, which no prediction equation can see.
WinSport Editorial & Research Standards
All formulas and reference values used in this tool are sourced from peer-reviewed scientific literature and validated clinical guidelines. Our research team continuously audits each calculator for accuracy and updates methodology as new evidence emerges.
Explore More Tools
View All Tools- Vitamin D CalculatorEstimate your sun exposure needs to reach optimal Vitamin D levels based on skin type and latitude.
- Magnesium Calculator — Daily Requirement for Sleep and RecoveryCheck your Magnesium intake and find your daily requirement to reduce anxiety and improve sleep.
- Iron Needs CalculatorEspecially important for female athletes. Calculate your daily iron requirement to prevent anemia.
Get the weekly research brief
One email per week. New calculators, methodology updates, and sports science summaries.