Energy calculation guide
BMR Formula Explained: Why Age, Height, Weight And Sex Matter
The Mifflin–St Jeor equation estimates resting energy from weight, height, age and a sex-specific constant. Here is the equation used by our BMR calculator and a worked example.
Updated 16 October 2026 • Prepared by Tools by Layna Editorial Team
The equation and required units
Use weight in kilograms, height in centimetres and age in years. The simplified equations from the original paper are:
- Male equation: resting kcal/day = 10 × weight + 6.25 × height − 5 × age + 5.
- Female equation: resting kcal/day = 10 × weight + 6.25 × height − 5 × age − 161.
The original research predicts resting energy expenditure (REE), measured by indirect calorimetry in healthy adults. Calculators often label this resting estimate “BMR”, although basal and resting measurement conditions are not identical. The output is a prediction, not a measurement of your metabolism.
Worked example: 80 kg, 180 cm, age 35
Using the male equation: 10 × 80 + 6.25 × 180 − 5 × 35 + 5 = 800 + 1,125 − 175 + 5 = 1,755 kcal/day.
Using the female constant with the same numerical inputs gives 1,589 kcal/day. The 166 kcal difference comes from the equation’s two constants; it is not an individual measurement of a biological difference.
Why each input changes the estimate
| Input | Role in this equation |
|---|---|
| Weight | Each additional kilogram adds 10 kcal/day to the model, with other inputs held constant. |
| Height | Each additional centimetre adds 6.25 kcal/day to the model. |
| Age | Each additional year subtracts 5 kcal/day from the model. |
| Sex-specific constant | The published simplified equation uses +5 or −161. It does not directly measure hormones or body composition. |
These are mathematical coefficients fitted to a study population. They do not mean changing one input causes exactly that change in every person’s measured energy use. If the two sex categories do not fit your circumstances, particularly with hormonal treatment, the calculator cannot resolve that uncertainty.
BMR is not maintenance intake
The 1,755 kcal example excludes the full day’s activity and the wider daily energy requirement. An illustrative activity multiplier of 1.55 gives 1,755 × 1.55 ≈ 2,720 kcal/day, but choosing a multiplier introduces another assumption. The main result in the BMR Calculator is the resting estimate; its activity tiles are additional scenarios.
Do not treat the resting estimate as a prescribed food intake or subtract an aggressive deficit from it. The original study included adults aged 19–78, so this page does not extend the equation to children or use it as a pregnancy or illness treatment method.
Frequently asked questions
Why does my watch show a different resting number?
A watch can use a different equation, profile or accumulated sensor data. Different estimates do not establish which one equals your measured resting expenditure.
Do I enter height in metres?
No. This version uses centimetres. Enter 180 for 1.80 metres; entering 1.80 directly changes the equation substantially.
Does the result include exercise?
The resting result does not. Daily maintenance estimates add activity assumptions, which are shown separately in the calculator and TDEE tools.
Related calculators and guides
Sources and method
- Mifflin et al. (1990): original resting-energy prediction study — Simplified equations, units and study population.
General explanation of a prediction equation. A clinician or accredited dietitian can assess cases where a general adult estimate is unsuitable.