Free BMR calculator
What does your body burn doing nothing?
Your basal metabolic rate, worked out three different ways — Mifflin–St Jeor, Harris–Benedict and, if you know your body fat, Katch–McArdle — shown side by side so you can see how far apart they land on the same body. It runs in your browser: no account, nothing sent anywhere.
General wellness guidance, not medical advice. If you are pregnant, breastfeeding, under 18, or managing a medical condition, talk to a doctor or a registered dietitian before acting on any of these numbers.
The three equations
| Equation | Formula | When to use it |
|---|---|---|
| Mifflin–St Jeor (1990) | 10 × kg + 6.25 × cm − 5 × age + 5 (men), − 161 (women) | The default for almost everyone, and what the NutriLog AI app uses |
| Harris–Benedict (revised 1984) | 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age (men) | Still widely quoted; usually reads high |
| Katch–McArdle | 370 + 21.6 × lean mass in kg | Best if you know your body fat and are very lean or very heavy |
Notice what Katch–McArdle does not need: your sex. It does not need it because lean mass already carries that information. That is also why it is the better equation at the extremes — a bodybuilder and a sedentary person of the same height and weight have very different resting costs, and only the lean-mass equation can see the difference.
What to do with the number
Not eat it. BMR is an input, not a target. Multiply it by an activity factor to get maintenance calories — that is the TDEE calculator, and it is the number you actually plan meals against. From there, the deficit calculator turns it into a target and a date, and the macro calculator splits it into grams.
And treat all of it as a starting hypothesis. A ten per cent error either way is normal, which on a 1,600 kcal BMR is 160 calories before you have multiplied anything. The correction comes from watching your real weight trend for two to three weeks — which is the loop NutriLog AI runs for you, re-refining the target every time you log a weight.
Common questions
What is BMR?
Basal metabolic rate: the energy your body spends doing nothing at all — heartbeat, breathing, temperature, brain, tissue repair. For most people it is sixty to seventy per cent of everything they burn in a day, which surprises anyone who assumes exercise dominates. It does not.
Which BMR formula is most accurate?
Mifflin–St Jeor for most people. It was validated against indirect calorimetry in the 1990s and it beats the older Harris–Benedict equation, which tends to read high — you can see that gap yourself in the comparison above, on your own numbers.
The exception is if you know your body-fat percentage and are either very lean or carrying a lot of fat. Then Katch–McArdle is better, because it works from lean mass and height-and-weight equations have no way to tell muscle from fat.
What is the difference between BMR and TDEE?
BMR is complete rest. TDEE is BMR multiplied by an activity factor, so it includes moving, working and training. You eat against TDEE, never against BMR — eating at your BMR while living a normal life is a large accidental deficit. Use the TDEE calculator for the number you actually eat to.
Should I eat my BMR to lose weight?
No. That is one of the most persistent bad ideas in dieting. Your BMR is what you burn lying still; living a normal day on top of it means the real deficit is far bigger than you intended, often 500 to 900 calories more. Size the deficit deliberately with the deficit calculator instead.
Does BMR slow down when you diet?
Somewhat, and less dramatically than the internet claims. A lighter body genuinely costs less to run, and there is a modest additional drop beyond what mass alone explains — mostly because you move less without deciding to. It is a hundred or two, not a broken metabolism, and it comes back as you eat more.
Why do the equations disagree?
Because they were fitted to different populations decades apart. Harris–Benedict came from a small early-twentieth-century sample and reads high for most modern bodies; Mifflin–St Jeor was fitted later on a broader group; Katch–McArdle sidesteps the problem by asking for lean mass directly. Showing the spread is more honest than picking one and calling it the answer.
Other free calculators
One equation, or a plan that corrects itself.
NutriLog AI starts from the same Mifflin–St Jeor number and then moves it as your real weight data arrives. Free, no ads, Android. Free on Google Play.