About the BMR Calculator
Basal metabolic rate is what you burn doing nothing at all — lying still, awake, fasted, at a comfortable temperature. It is the largest single part of what anyone burns in a day.
It also cannot be measured outside a laboratory. Every BMR figure you will ever see, including this one, is a regression fitted to a population. That is not a disclaimer to skip past; it is the most important thing about the number, and it is why this BMR calculator shows you three equations and how far apart they are rather than one figure to the nearest calorie.
How to Use the BMR Calculator
Enter age, sex, height and weight in metric or imperial.
Body fat percentage is optional. Supply it and Katch-McArdle becomes available, which is the most interesting of the three equations for reasons below.
Pick an equation, or leave it on Mifflin-St Jeor, which is the standard choice.
Step-by-Step Example
Male, 35, 180cm, 80kg.
Mifflin-St Jeor:
10 × 80 + 6.25 × 180 − 5 × 35 + 5
800 + 1125 − 175 + 5 = 1755About 1,755 calories a day at rest. Harris-Benedict gives 1,825 on the same figures — a 4% disagreement, and neither of them is measuring this person.
The Three Equations
Mifflin-St Jeor (1990) is the current standard. Developed on a more representative population than its predecessor, and generally the most accurate across general populations.
Harris-Benedict (1919, revised 1984) is still widely quoted and tends to read higher. If a figure elsewhere looks generous, this is often why.
Katch-McArdle works from lean body mass:
BMR = 370 + 21.6 × lean mass in kgWhy Katch-McArdle Has No Sex Term
This is the detail worth understanding, because it explains what the sex term in the other equations is actually doing.
Look at the formula again. No sex. No age either.
That is not an oversight. The sex term in Mifflin-St Jeor — a flat 166 calories, identical at every height, weight and age — is a proxy for body composition. Men carry more lean mass on average, and lean tissue is what burns energy. The equation cannot see your lean mass, so it uses sex to guess at it.
Once you actually know the lean mass, the proxy has nothing left to explain.
The consequence is testable and the calculator tests it: two people with identical lean mass get an identical figure, whatever their sex or age. A 35-year-old man of 80kg at 20% body fat and a 52-year-old woman of 80kg at 20% body fat both come out at 1,752 calories, because both carry 64kg of lean tissue.
That is either a feature or a warning, depending on how good your body fat figure is. Katch-McArdle is the best of the three when that number is reliable and the worst when it is a guess, because the entire equation rests on it.
The Honest Answer Is a Range
These regressions carry a standard error of roughly ±10% on the populations they were fitted to.
Point estimate: 1,755
Realistic range: 1,580 to 1,931That is a band about 350 calories wide — more than a meal. Presenting 1,755 as though the last digit meant something implies a precision that does not exist.
The calculator also shows the gap between the equations, usually around 4%, for the same reason. When three serious attempts at the same quantity disagree by 70 calories, that disagreement is information about how much any of them can tell you.
BMR Is Not a Target
A common and consequential misreading: BMR is not the number to eat at.
It is what you would burn in a coma. Add everything you actually do — walking to the kitchen, standing, working, training — and the daily figure is 20% to 90% higher. That total is TDEE, and it is the number that matters for eating.
Eating at your BMR while living an ordinary life is already a substantial deficit, often 500 calories or more, without having decided to be in one.
Why Your BMR Falls as You Lose Weight
It does, and it is worth expecting rather than being alarmed by.
A smaller body costs less to run. Some of what you lose is lean tissue, not just fat. Both push the figure down.
Run the calculation again at your new weight and the lower number is usually the whole explanation for a deficit that worked for two months and stopped in the third. It is ordinary arithmetic rather than a metabolism that has broken.
Understanding Your Result
Resting burn is the point estimate from your chosen equation.
The honest range applies the ±10% and is the figure to actually carry around.
All three equations shows what each one says.
How far apart they are gives the spread in calories and as a percentage.
Worth knowing flags a large disagreement, a fallback, or what a body fat figure would add.
When Should You Use This Calculator?
As the first step toward TDEE. BMR alone answers very little.
To sanity-check a figure from elsewhere. If an app is 300 calories away from all three equations, something is off.
After significant weight change. The figure moves, and so does everything built on it.
When you have a reliable body fat measurement. That is when Katch-McArdle earns its place.
Common Mistakes
Eating at BMR. It is the floor, not the target. TDEE is the target.
Treating the figure as measured. It is a population regression with ±10% of error.
Trusting Katch-McArdle with a guessed body fat figure. The whole equation rests on that one input.
Using Harris-Benedict because it gives a bigger number. It is the older equation and generally reads high.
Assuming a falling BMR means a damaged metabolism. A smaller body burns less. That is all.
Recalculating instead of observing. After three weeks, the scale knows more than any equation.
These equations are estimates fitted to populations and can be well out for any individual, particularly anyone very muscular, very lean, elderly or unwell. Nothing here is medical or dietary advice — speak to a doctor or registered dietitian before making significant changes to what you eat.