How this works
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — keeping organs running, brain firing, and core temperature steady. It does not include movement, exercise, or digestion. BMR is the foundation for any calorie target: multiply it by an activity factor (typically 1.2–1.9) to get your total daily energy expenditure (TDEE), then add or subtract calories to gain or lose weight.
The formula
This is the Mifflin-St Jeor equation, the modern reference standard since 1990 — about 5% more accurate than the older Harris-Benedict for typical adults. The constant differs by sex (+5 for men, −161 for women) to reflect average lean-mass differences. Result is calories per day.
Example calculation
- A 30-year-old man, 175 cm, 75 kg, sedentary office worker.
- BMR = 10 × 75 + 6.25 × 175 − 5 × 30 + 5 = 750 + 1093.75 − 150 + 5 ≈ 1,699 kcal/day.
- Multiply by 1.2 (sedentary) for daily energy needs: 1,699 × 1.2 ≈ 2,040 kcal/day to maintain weight.
Frequently asked questions
What's the difference between BMR and TDEE?
BMR is what you burn at rest, doing nothing. TDEE (Total Daily Energy Expenditure) is what you burn in a real day, with movement, exercise, and digestion included. TDEE = BMR × activity multiplier: roughly 1.2 sedentary, 1.375 light exercise, 1.55 moderate, 1.725 hard, 1.9 very hard. Diet plans always target TDEE, not BMR.
How accurate is the Mifflin-St Jeor formula?
Within about ±10% for a typical adult between 19 and 78. It is less accurate for very lean athletes (who have higher BMR than the formula predicts) and people with significant obesity (where formulas based on lean mass do better). Treat the result as a starting point — track weight for a few weeks and adjust calorie targets up or down by 100–200 kcal until weight changes match your goal.
Why does my BMR drop as I age?
Two reasons: lean muscle mass tends to decline by ~3–8% per decade after 30, and metabolically active organ tissue shrinks slightly. The formula bakes in a 5 kcal/year drop, which captures the average — strength training and adequate protein can soften it considerably.
Should I eat exactly my BMR worth of calories to lose weight?
No — eating below your BMR is risky over time. The healthy target is a moderate deficit from your TDEE: 300–500 kcal/day produces about 0.25–0.5 kg of weight loss per week. Going much lower can drop metabolic rate, costs muscle, and is rarely sustainable. Aggressive cuts should be supervised by a clinician.