How Your Basal Metabolic Rate Is Calculated
The Mifflin-St Jeor equation explained step by step, and how BMR becomes a real daily calorie target.
Basal Metabolic Rate (BMR) is the calorie floor your body needs just to keep functioning — breathing, circulating blood, maintaining body temperature — with zero physical activity factored in yet.
The Mifflin-St Jeor equation
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s
where s is +5 for men and −161 for women. This equation, published in 1990, is now the most widely recommended BMR formula in clinical nutrition, having outperformed the older Harris-Benedict equation in modern validation studies.
Worked example
A 28-year-old woman weighing 65kg at 165cm: BMR = (10×65) + (6.25×165) − (5×28) − 161 = 650 + 1031.25 − 140 − 161 = 1,380.25 kcal/day. That's the estimated calorie burn from simply existing for 24 hours, before a single step of walking or exercise is counted.
Turning BMR into a real target: TDEE
BMR alone isn't a useful calorie target for eating — almost nobody is completely sedentary for 24 hours. Multiplying BMR by an activity factor produces Total Daily Energy Expenditure (TDEE), the actually useful number:
- Sedentary (little/no exercise): × 1.2
- Lightly active (1-3 days/week): × 1.375
- Moderately active (3-5 days/week): × 1.55
- Very active (6-7 days/week): × 1.725
- Athlete (very intense daily training): × 1.9
That same 1,380.25 BMR, for a moderately active lifestyle, becomes a TDEE of 1,380.25 × 1.55 ≈ 2,139 kcal/day — the calorie level at which her weight would be expected to stay roughly stable.
Why the sex-specific offset exists
The +5/−161 difference isn't arbitrary — it approximates the average difference in body composition between men and women at the same weight and height, since men typically carry more lean muscle mass (which burns more calories at rest) relative to total body weight than women do on average. It's a population-level adjustment, not a claim about any specific individual.
Common mistakes to avoid
- Eating at or below your BMR long-term (rather than your TDEE) — this can cause the body to adapt by lowering metabolic rate further, working against sustainable weight loss
- Forgetting that BMR formulas are population averages with roughly ±10% individual variation — two people with identical stats can have genuinely different real metabolic rates
- Using BMR as if it already accounts for activity — it explicitly does not; that's what the activity multiplier is for
Calculate your own BMR with the BMR calculator, then get your full TDEE with the TDEE calculator or a direct calorie target with the calorie calculator.