BMR Calculator

BMR is the floor, not the target

Basal Metabolic Rate is the energy your body uses lying still in a warm room, fasted — running the heart, brain, kidneys and cell repair. Nobody burns only their BMR: every step, every meal digested and every shiver adds to it. Multiplying BMR by an activity factor gives Total Daily Energy Expenditure, which is the number that actually governs weight change. The multipliers above are the standard set.

Why Mifflin-St Jeor replaced Harris-Benedict

The Harris-Benedict equation dates from 1919 and was derived from a small sample of lean subjects. Mifflin-St Jeor, published in 1990, was validated on a broader modern population and predicts measured resting energy more accurately for most people — which is why clinical dietetics uses it as the default. Both are shown above; where they disagree, the Mifflin figure is the better estimate.

Every equation is an estimate

Predictive equations typically land within about 10% of measured BMR for most people, but individuals vary. Body composition matters: two people of identical height, weight, age and sex can have different BMRs if one carries more lean mass, because muscle is metabolically costlier than fat. If you know your body fat percentage, the Katch-McArdle equation based on lean mass will usually beat both equations here.

BMR is not a fixed number for life

It moves. Sustained calorie restriction lowers it somewhat as the body economises — part of why weight loss slows and why very aggressive deficits backfire. Building muscle raises it modestly. Age lowers it gradually, largely through lost lean mass rather than age itself, which is why resistance training in later decades protects metabolic rate more effectively than cardio alone. Recalculate after any significant change in weight or training, and treat a target that sits below BMR as a warning sign rather than a plan.

Frequently Asked Questions