How To Calculate Your Bmr
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What BMR actually measures
BMR is the energy cost of basic physiological processes: heart beating, lungs breathing, kidneys filtering, liver metabolizing, brain running, cells repairing themselves. Measured in a lab: 12 hours fasted, lying still, neutral temperature, awake.
Mifflin-St Jeor (current standard)
Published 1990, the best-validated formula for modern populations. Use this unless you have a specific reason not to.
Harris-Benedict (revised 1984)
Accurate to within ~10% for 82% of normal-weight people. Accuracy drops at the extremes of age and body composition.
Katch-McArdle (body-composition aware)
The older standard, still common in textbooks. Slightly overestimates for modern (heavier) populations.
What actually determines your BMR
Gives numbers ~100 kcal higher than Mifflin-St Jeor for the same person. Use only if you’re matching an existing study or device that uses it.
What doesn’t matter as much as people say
If you know your body fat percentage, this formula uses lean body mass and is more accurate for lean or muscular people whose fat-free mass deviates from average.
Metabolic adaptation
For bodybuilders and athletes, Katch-McArdle beats Mifflin by 100– 200 kcal. For sedentary people with higher body fat it slightly underestimates.
BMR and aging
Four factors explain most of the variance:
How to measure BMR for real
The most important concept most BMR calculators ignore: BMR is not a constant. Prolonged calorie restriction lowers BMR beyond what weight loss alone accounts for.
Limitations of any BMR formula
This is why plateaus happen and why “diet breaks” help. A 1–2 week return to maintenance partially reverses the adaptation. The Biggest Loser follow-up study is the most famous example — contestants had BMRs 400+ kcal below predicted even years later.
Why BMR matters even if you’re not dieting
The common claim is metabolism “slows” after 30 or 40. A 2021 Science paper tracking 6,400 people showed the truth is less dramatic:
Common mistakes
Most “metabolism slowed” between 30 and 50 is actually activity and muscle loss, not a BMR change.
Run the numbers
If accuracy matters (clinical, research), the gold standards are: