How the BMR calculator works
What BMR actually is
Basal metabolic rate is the energy of simply existing: brain, heart, liver, kidneys, and cell maintenance at true rest. It runs 60–75% of total daily burn, scales with lean mass (which is why strength training raises it), and falls ~2% per decade after 30 mostly via muscle loss — not mystique.
Mifflin-St Jeor, and why it wins
The 1990 Mifflin-St Jeor equation (10×kg + 6.25×cm − 5×age + sex term) beat the older Harris-Benedict in validation studies against measured metabolic carts — within ~10% for most adults. It is the standard used in clinical practice and on this page; the older formulas run hot by ~5%.
From BMR to a real daily number
BMR × activity factor estimates TDEE, and the honest activity factor is where most error enters: "moderate" means 3–5 genuine workouts plus an otherwise ordinary day, not two walks and good intentions. Tracking intake against weight trends for two weeks calibrates the estimate better than any questionnaire.