The Mifflin-St Jeor formula
- Men: 10×weight(kg) + 6.25×height(cm) − 5×age + 5
- Women: 10×weight(kg) + 6.25×height(cm) − 5×age − 161
Worked example: a 40-year-old man weighing 80 kg at 178 cm gets 10×80 + 6.25×178 − 5×40 + 5 = 800 + 1,112.5 − 200 + 5 ≈ 1,718 kcal per day at complete rest.
The equation was published in 1990 in the American Journal of Clinical Nutrition and displaced the 1919 Harris-Benedict formula as the clinical default, because it matches measured resting expenditure more closely in modern populations — Harris-Benedict was built on a small, unrepresentative sample and tends to run high.
BMR, RMR and TDEE
Three terms that get used interchangeably and should not be.
BMR is measured under strict conditions: fully rested, fasted, at a comfortable temperature, immediately on waking. RMR (resting metabolic rate) is measured under less strict conditions and runs slightly higher — perhaps 10% — which is why the two numbers differ in the literature. Most consumer calculators, including this one, use equations derived from resting measurements and label the output BMR.
TDEE is BMR plus everything else you do: moving, exercising, and digesting. It is the figure you actually eat around to maintain weight, and it is what our TDEE calculator gives you.
What determines your BMR
- Body size — a larger body has more tissue to maintain. This is the biggest single factor.
- Lean mass — muscle and organs are metabolically active in a way fat is not, which is why two people of the same weight can differ.
- Age — resting expenditure declines gradually through adult life, mostly because lean mass declines.
- Sex — men average higher, largely because they average more lean mass.
- Genetics and thyroid function — real, individually variable, and mostly outside your control.
Can you raise it?
Somewhat, and slowly. Building muscle through resistance training increases lean mass and therefore resting expenditure — but the effect is more modest than commonly claimed. Resting muscle burns roughly 13 kcal per kilogram per day, so adding a genuinely impressive 5 kg of muscle raises BMR by something like 65 kcal a day. Real, worth having, and not a transformation.
The stronger arguments for resistance training are elsewhere: it protects muscle during fat loss, improves insulin sensitivity, maintains bone density and function with age, and lets you eat more while staying the same size. Treat the metabolic bump as a bonus.
Prolonged aggressive dieting does reduce resting expenditure by somewhat more than the weight loss alone predicts — a phenomenon known as adaptive thermogenesis. It is a real effect and a further argument for moderate rather than severe deficits.
Do not eat below your BMR
A common and counterproductive mistake. Your deficit should be taken from your TDEE, which is meaningfully higher than BMR. Eating below the energy your body needs at complete rest, while also moving around and training, tends to produce disproportionate muscle loss, persistent fatigue, poor training quality, and difficulty meeting basic nutritional requirements.
For most adults a target of 300–500 calories below TDEE lands comfortably above BMR. If your arithmetic puts a sensible deficit below your BMR, that usually means your activity level is genuinely low and the better lever is adding movement rather than cutting food further.
When a lean-mass equation does better
Enter a body-fat percentage and the result adds a second equation. Cunningham (1980) re-analysed the original Harris and Benedict subjects and found that once lean body mass was known, it was the single predictor that mattered: sex and age added little. The whole equation is 500 + 22 × lean mass in kilograms.
That makes it the better choice exactly where Mifflin-St Jeor is weakest. A muscular person carries more metabolically active tissue than their weight implies, so a weight-based formula tends to under-estimate them. Someone with a high body-fat percentage carries less, and it tends to over-estimate.
The catch is that Cunningham is only as good as the body-fat figure you give it. A tape or smart-scale reading can easily be several points out, and for an 80 kg person every 5 points of error is 4 kg of lean mass, or 88 kcal in the result. If your body-fat number is a guess, the difference between the two equations is mostly noise.