What BMR means
BMR is the energy your body uses just to stay alive — breathing, circulation, keeping warm — if you lay still all day. It is usually 60–70% of everything you burn. Multiply it by an activity factor to get the calories you actually need (the table in the result).
Which formula to pick
Mifflin-St Jeor is the one most dietitians use today — it replaced Harris-Benedict because it matches measured BMR more closely for most adults. Harris-Benedict (the 1984 Roza-Shizgal revision used here) tends to run a little high, especially for people who are overweight. Katch-McArdle takes your weight and body-fat percentage together to work out lean mass, so it's only as good as the body-fat number you feed it — useful if you already know that number from a caliper or scan, not worth guessing at otherwise.
Common questions
Why is my BMR different from what my fitness watch shows?
Wearables estimate resting energy from heart-rate and movement patterns, not a formula — the two rarely land on the exact same number. A 100–150 calorie gap either way is normal; treat both as estimates, not lab measurements.
Does more muscle actually raise BMR by much?
Yes, but less than people expect — a kilogram of muscle burns roughly 13 calories a day at rest, versus about 4.5 for a kilogram of fat. Someone who's added 5kg of muscle is looking at maybe 40 extra calories a day from that alone, not hundreds.
Is this accurate for a very lean or very heavy body?
Less so. These formulas were built from average population data, so they get shakier at the extremes — very low body fat or a BMI well above 35. Katch-McArdle (with a real body-fat number) usually tracks better than the other two in those cases.
Should I eat at my BMR to lose weight?
No — BMR is what you'd burn lying still all day, not a target. Your maintenance calories (BMR × activity factor, shown in the results table) is the number to diet against; eating below your actual BMR for long stretches tends to backfire.
For general information only — not medical advice. Talk to a doctor about anything that concerns you.