BMR Calculator: How It Works
Basal metabolic rate is the energy your body uses at complete rest simply to stay alive — heartbeat, breathing, temperature, brain and organ function. It is the largest single component of daily energy use for most people, and the base that every calorie target is built on.
The Mifflin-St Jeor equation
This is the equation most dietitians use, having been validated as more accurate than the older Harris-Benedict formula for contemporary populations.
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
A 32-year-old woman, 165 cm, 62 kg: (10 × 62) + (6.25 × 165) − (5 × 32) − 161 = 620 + 1031.25 − 160 − 161 = 1,330 kcal/day.
From BMR to daily expenditure
BMR is what you would burn lying still all day. Total daily energy expenditure multiplies it by an activity factor:
| Activity level | Multiplier | Describes |
|---|---|---|
| Sedentary | 1.2 | Desk work, little deliberate exercise |
| Lightly active | 1.375 | Light exercise 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extremely active | 1.9 | Physical job plus daily training |
The woman above, moderately active: 1,330 × 1.55 ≈ 2,060 kcal/day.
Where the energy actually goes
| Component | Share of daily total |
|---|---|
| Basal metabolic rate | 60% – 70% |
| Thermic effect of food (digestion) | ≈ 10% |
| Non-exercise activity (fidgeting, walking, posture) | 15% – 30% |
| Deliberate exercise | 5% – 10% for most people |
Deliberate exercise is usually the smallest slice, and non-exercise activity varies more between individuals than exercise does. This is why two people with identical training schedules can have meaningfully different daily expenditure.
What raises and lowers BMR
- Lean mass is the largest modifiable factor. Muscle is metabolically active tissue; more of it raises BMR modestly but permanently.
- Body size. Larger bodies require more energy to maintain, which is why BMR falls as weight falls.
- Age. BMR declines gradually through adulthood, largely tracking loss of lean mass rather than age itself.
- Sex. The difference in the equations reflects average differences in lean mass and body composition.
- Prolonged energy restriction reduces BMR beyond what weight loss alone predicts — a genuine adaptation, though its size is often exaggerated.
Accuracy, honestly
Mifflin-St Jeor estimates BMR within about 10% for roughly 80% of people. That is a useful starting point and not a measurement. Individual variation comes from body composition, thyroid function, genetics and medication, none of which the equation sees. If you have very high or very low body fat, an equation using lean mass — such as Katch-McArdle — will usually be more accurate.
The practical approach: use the estimate as a starting figure, then track actual intake and weight over two to three weeks and adjust based on what happens. Your own data beats any formula.
This page is general information, not medical or dietary advice. Very low calorie intakes carry real health risks and should only be undertaken with medical supervision. Speak to a doctor or registered dietitian before making significant changes.