Basal metabolic rate (BMR) calculator

Basal metabolic rate is the energy your body spends simply staying alive: breathing, pumping blood and maintaining temperature.

It accounts for 60% to 70% of total daily expenditure in a sedentary person.

The two most used formulas are shown. Mifflin-St Jeor is currently recommended as more accurate for the general population; Harris-Benedict is the classic and usually returns slightly higher figures.

What it actually measures

Basal metabolic rate is the energy your body uses at complete rest to maintain vital functions: breathing, circulation, temperature, brain activity and cell renewal. It is what you would burn lying awake all day without moving. In most people it accounts for sixty to seventy per cent of total expenditure.

Why the formulas fail at the extremes

Mifflin-St Jeor and Harris-Benedict were fitted on populations of average build. In people with substantial muscle they underestimate expenditure, because muscle burns more at rest than fat, and in severe obesity they overestimate it. The Katch-McArdle formula, which starts from body fat percentage, usually works better in those cases, though it requires a reliable measurement.

What genuinely changes metabolism

Muscle mass is the most relevant modifiable factor: keeping or building it sustains resting expenditure. Age lowers it slowly, largely through the associated loss of muscle. Prolonged very restrictive diets also reduce it, which explains the plateau after weeks in deficit.

An estimate, not a measurement

The real figure is obtained by indirect calorimetry in a laboratory. Any formula gives an indicative range with a margin that can exceed ten per cent.