How to Calculate Your Basal Metabolic Rate

Estimates the energy your body burns at complete rest over a day, using the Mifflin-St Jeor equation, together with the daily total once activity is taken into account.

Explanation

Your basal metabolic rate is the energy you burn doing absolutely nothing: breathing, pumping blood, holding your temperature, running your organs. Spend the whole day in bed and you still spend this much.

\text{Men}: \quad 10W + 6.25H - 5A + 5

\text{Women}: \quad 10W + 6.25H - 5A - 161

Here $W$ is weight in kilograms, $H$ height in centimetres and $A$ age. This is the Mifflin-St Jeor equation, the one dietitians' associations now recommend.

Example

A 30-year-old man, 170 cm, 65 kg.

10 \times 65 + 6.25 \times 170 - 5 \times 30 + 5 = 650 + 1062.5 - 150 + 5 = 1567.5\ \text{kcal/day}

Over 1500 kilocalories just to lie there. The daily total comes from multiplying by an activity factor.

Sedentary, desk-bound — ×1.2 → 1881 kcal
Moderately active — ×1.55 → 2430 kcal
Very active — ×1.725 → 2704 kcal

The arithmetic of dieting

Burning a kilogram of body fat takes a deficit of roughly 7200 kcal.

At 500 kcal a day, that is $7200 \div 500 = 14.4$ days: a fortnight per kilogram. This one calculation is enough to show why "lose 5 kg this month" is not a serious plan.

Watch out

This is an estimate. Two people of identical build can differ by 10% either way.

The formula never looks at muscle. Muscle burns more energy than fat does, so a muscular person tends to run above the prediction and someone carrying more fat below it.