What BMR tells you
Your basal metabolic rate is the energy cost of being alive: your heart beating, lungs breathing, brain thinking, cells repairing. For most people it is the largest slice of daily expenditure, larger than training, which surprises almost everyone.
The Mifflin-St Jeor equation estimates it from weight, height, age and sex: 10 × kg + 6.25 × cm − 5 × age + 5 for men, or − 161 for women.
What BMR does not tell you
It is not a calorie target. Nobody lives at complete rest. To find what you actually burn, multiply BMR by an activity factor to get your TDEE, and set deficits or surpluses from there.
It also cannot see body composition. Two people at 80 kg, one lean and muscular, one not, have different resting metabolisms. If you know your body-fat percentage, lean-mass formulas are more precise.
The floor you should respect
- Do not eat below BMR for extended periods, even when cutting aggressively.
- If your calculated target falls below ~1,200 kcal (women) or ~1,500 kcal (men), the deficit is too aggressive; reduce it or increase activity instead.
- Metabolic adaptation is real: after weeks in a deficit, expenditure drops 5–15%. A diet break at maintenance every 6–10 weeks helps.