What the formula calculates
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain vital functions: breathing, circulation, cell production, and temperature regulation. The Mifflin-St Jeor equation, published in 1990, is the most accurate widely-used formula: For men: BMR = 10W + 6.25H - 5A + 5. For women: BMR = 10W + 6.25H - 5A - 161. Where W = weight in kg, H = height in cm, and A = age in years.
TDEE and activity multipliers
BMR represents calories at complete rest. Real daily calorie expenditure (TDEE) multiplies BMR by an activity factor: Sedentary (desk job, no exercise) × 1.2. Lightly active (1-3 days/week exercise) × 1.375. Moderately active (3-5 days/week) × 1.55. Very active (hard exercise 6-7 days/week) × 1.725. Extra active (physical job + training) × 1.9.
Worked example
A 35-year-old woman weighing 68 kg at 165 cm: BMR = 10×68 + 6.25×165 - 5×35 - 161 = 680 + 1031.25 - 175 - 161 = 1375 kcal/day. If moderately active, TDEE = 1375 × 1.55 = 2131 kcal/day.
Limitations
Predicted BMR equations have standard errors of approximately 100-150 kcal per day. Individual metabolic rates vary based on body composition (muscle burns more at rest than fat), genetics, thyroid function, and adaptation to caloric restriction. BMR equations provide useful starting estimates, not precise individual measurements.