What BMR measures
Your Basal Metabolic Rate is the energy your body spends just staying alive — circulating blood, breathing, maintaining body temperature and repairing cells. Even someone who never leaves bed burns this much. This calculator uses the Mifflin-St Jeor equation, the predictive formula recommended by the American Dietetic Association for the general adult population:
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Worked example
A 30-year-old man, 70 kg (154 lb) and 170 cm (5′7″): 10 × 70 = 700, plus 6.25 × 170 = 1,062.5, minus 5 × 30 = 150, plus 5 gives BMR ≈ 1,618 kcal/day. If he trains moderately 3–5 times a week, his daily burn is roughly 1,618 × 1.55 ≈ 2,507 kcal.
Mifflin-St Jeor vs Harris-Benedict
The older Harris-Benedict equation (1919, revised 1984) was built on early-20th-century body-composition data and tends to overestimate modern resting metabolism by around 5%. Mifflin-St Jeor (1990) was derived from a larger, more representative sample and predicts measured resting energy expenditure within 10% for the most people, which is why it has become the clinical default. Both are estimates — a metabolic cart measurement is the only way to know your true number.
Limitations
No weight-and-height formula sees your body composition. Very muscular people burn more than predicted and people with low lean mass burn less; thyroid conditions, some medications and severe dieting history also shift the true figure. Treat the result as a starting point, watch your weight trend for two to three weeks, and adjust intake in small steps.