What BMR actually is (and isn't)
Your basal metabolic rate (BMR) is the energy your body spends just staying alive: keeping your heart beating, your brain running, your organs working and your temperature stable. Even if you spent an entire day motionless in bed, you would still burn your BMR — for most adults that's somewhere between 1,200 and 2,000 kcal, and it typically accounts for 60–75% of everything you burn in a day.
What BMR is not is the number of calories you should eat. Once you add walking, training, fidgeting and digestion on top, your total daily burn (TDEE) lands well above your BMR. Treat BMR as the foundation of the calculation, not the target itself — the calorie calculator turns it into a daily eating target.
The equations
This calculator's headline number uses Mifflin-St Jeor (1990) — the equation the Academy of Nutrition and Dietetics recommends, because it tracks measured resting metabolism more closely than the older Harris-Benedict formula across modern populations:
If you know your body-fat percentage, the calculator also shows the Katch-McArdle estimate, which works from lean body mass instead of total weight:
Which one should you trust? If your body-fat number comes from a reliable measurement (DEXA, calipers, or a consistent tape protocol like our body fat calculator), Katch-McArdle is usually the better pick — especially if you carry more muscle than average, which Mifflin-St Jeor can't see. If you're guessing at your body fat, stick with Mifflin-St Jeor: a wrong body-fat input skews Katch-McArdle more than sex, height and age skew Mifflin.
What changes your BMR
Four things move the number, and only one of them is fully in your control:
- Body size. More body means more tissue to run. This is why BMR falls as you lose weight — a lighter body simply costs less to operate.
- Muscle mass. Lean tissue burns more at rest than fat tissue. It's the main lever you can pull: consistent strength training and adequate protein nudge your resting burn up over months and years.
- Age. BMR drifts down slowly with age — roughly 1–2% per decade in adulthood, much of it explained by gradual muscle loss rather than age itself. Training well is partly a metabolism-preservation strategy.
- Dieting adaptation. During a prolonged calorie deficit the body trims its energy use — a real effect called adaptive thermogenesis, typically 5–15% beyond what the weight loss alone predicts. It's why the same formula overestimates slightly late in a diet, and why targets need periodic re-checks.
From BMR to daily calories
To plan your food, multiply your BMR by an activity factor to get your TDEE — total daily energy expenditure. That's the maintenance number every fat-loss or muscle-gain plan is built from, and getting the activity multiplier honest is where most people go wrong. Do that step properly with the daily calorie (TDEE) calculator — it starts from the same equations on this page and adds fat-loss and lean-gain targets.
And once you have a target, the hard part is hitting it every day. If you train with a coach, the CoachRight client app logs your food (with AI nutrition-label scanning) so your coach sees exactly how intake matches the plan. Coaches: you can run your whole client roster — targets, programs, check-ins — from one place; start a free 14-day trial.