What BMR is
Basal Metabolic Rate is the number of calories your body burns at complete rest — lying still, fasted, in a temperate room — just to keep core functions running. About 60–75% of total daily calories go to BMR in a typical sedentary adult. The rest is the thermic effect of food (digestion, ~10%) and physical activity (everything else).
BMR is the foundation of every diet and weight-management plan. To lose, gain, or maintain weight you need an accurate baseline of how many calories your body uses without any movement.
The Mifflin-St Jeor equation
Several BMR formulas exist; the most widely recommended is Mifflin-St Jeor (1990), which the American Dietetic Association considers the most accurate for non-obese and obese adults alike.
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) − 161The only difference between sexes is the constant — reflecting the higher proportion of lean mass in average male physiology.
A worked example
A 35-year-old woman, 165 cm, 65 kg:
BMR = 10 × 65 + 6.25 × 165 − 5 × 35 − 161
= 650 + 1031.25 − 175 − 161
= 1345 kcal/daySo even completely sedentary, she would burn about 1,345 calories per day. Any movement, food digestion, or thermoregulation adds to that.
From BMR to TDEE — total daily energy expenditure
BMR is the floor. To estimate maintenance calories — the amount that keeps weight stable — multiply BMR by an activity factor:
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little exercise | 1.2 |
| Lightly active | Light exercise 1–3 days/week | 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | 1.55 |
| Very active | Hard exercise 6–7 days/week | 1.725 |
| Extra active | Hard daily exercise + physical job | 1.9 |
The example BMR of 1,345 × 1.55 (moderately active) gives a TDEE of about 2,085 kcal/day. For weight loss, subtract 500/day (≈1 lb/week). For gain, add 250–500/day.
Other BMR formulas and when to use them
- Mifflin-St Jeor (1990) — default. Most accurate for the general population.
- Harris-Benedict (revised 1984) — older, slightly overestimates in lean individuals. Still widely cited.
- Katch-McArdle — uses lean body mass instead of total weight. More accurate for very lean or very muscular people if you know your body-fat percentage.
- Cunningham (1980) — also lean-mass based, popular among athletes.
- WHO/FAO/UNU (1985) — separate equations for each age bracket. Used in some clinical settings.
Why your real BMR may differ
- Genetics: documented metabolic variation of ±10–15% even between same-sized adults.
- Lean mass: muscle burns more calories at rest than fat. Athletes can have BMRs 20% above the formula.
- Thyroid function: hyperthyroidism raises BMR; hypothyroidism lowers it. Diagnosed conditions need clinical adjustment.
- Adaptive thermogenesis: after prolonged dieting, BMR drops 5–15% beyond what weight loss alone predicts ("metabolic adaptation").
- Ambient temperature: cold environments raise BMR; thermoneutral environments lower it.
- Age: the formula already accounts for the gradual drop with age (~1–2% per decade after 20), but individual rates vary.
For most people the formula is accurate within ±200 kcal — close enough to set a starting point. Adjust based on actual weight change over 2–4 weeks of consistent eating.
How to measure BMR more accurately
For research-grade numbers, BMR is measured by indirect calorimetry — a mask captures inhaled oxygen and exhaled carbon dioxide while the subject lies still after a 12-hour fast. The ratio reveals the substrate (fat vs carbohydrate) being burned and the calorie output. Commercial gyms and metabolic clinics offer this test for around $100–200.
Wearables (Apple Watch, Garmin, Fitbit) estimate energy expenditure but combine BMR + activity; they are best used to track changes over time rather than for absolute calibration.