BMR Calculator

The energy your body spends just staying alive — heartbeat, breathing, brain activity, temperature regulation, cell turnover — measured at complete rest. It's typically 60–75% of everything you burn in a day, which surprises people: for most bodies, existing costs far more than exercising. Everything you actually do (walking, working, training, even digesting) stacks on top of BMR to form your total daily burn (TDEE).

Mifflin-St Jeor, unless you know your body fat percentage. The American Dietetic Association's review of prediction equations found it most accurate across the general population, which is why it replaced Harris-Benedict as the standard. If you have a real body fat measurement, Katch-McArdle is better still, because it computes from lean mass — the tissue doing the burning — instead of inferring it from height, weight, age, and sex.

Each was fit to a different study population with different statistical assumptions, and predicting an individual from population data always carries error — the honest expectation is roughly ±10% for any of them. The spread you see between the formulas is a useful reminder that they're all estimates. If precision matters (medical settings, serious coaching), indirect calorimetry — the breath test — is the only actual measurement.

The one meaningful lever is muscle mass — each pound of muscle burns a small but real amount at rest, and resistance training over months adds up. The things marketed as metabolism boosters (green tea, cold showers, spicy food, meal timing) test out to rounding errors. BMR also drifts down with age (roughly 1–2% per decade after 20) and adapts downward somewhat during aggressive dieting — one more argument for moderate deficits.