What it is
The BMR Calculator works out your basal metabolic rate, which is the energy your body spends just staying alive: breathing, circulating blood, keeping its temperature, running your brain. It then multiplies that by your activity level to estimate what you actually burn in a day.
The formula it uses
The calculation is the revised Harris-Benedict equation (Roza and Shizgal, 1984), which takes weight, height, age and sex. It is one of the oldest and best-known formulas, and it holds up well for people in a typical body composition range.
The result is then multiplied by an activity factor, from sedentary through to very active. That second number, your total daily expenditure, is the one people actually want.
How to use it
- Enter sex, age, height and weight.
- Pick your activity level, honestly. Most people overestimate this, and it is the single biggest source of error in the final figure.
- Read both numbers: the BMR itself and the daily total with activity applied.
How much to trust the number
It is an estimate, and the honest error bar is wider than the tidy figure suggests.
The formula does not know your body composition, and muscle burns more at rest than fat does. Two people with identical height, weight, age and sex can have genuinely different metabolic rates. Illness, medication and thyroid function all move the number as well.
Treat it as a starting point rather than a target: use it for a couple of weeks, watch what actually happens to your weight, and adjust from the real result rather than from the prediction.
This is informational content and does not replace a doctor or a registered dietitian, especially if you have a health condition or are planning a large change in how you eat.
Related: the BMI calculator.