Basal Metabolic Rate Calculator

Basal metabolic rate is the energy your body burns at rest to maintain core function — about 60–75% of total daily expenditure for sedentary adults. The Harris-Benedict equation (1919, revised 1984) and the Mifflin-St Jeor equation (1990) are the two clinical standards. Mifflin-St Jeor is generally more accurate for the modern population.

The calculator returns BMR by both formulas plus an activity-adjusted TDEE estimate.

BMR is measured under strict conditions: post-absorptive (12+ hours fasted), thermally neutral environment, fully rested, awake but motionless. Most consumer calculators report something between true BMR and the slightly higher resting metabolic rate (RMR), measured under less restrictive conditions. The Mifflin-St Jeor equation is BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5 (for men) or − 161 (for women). Harris-Benedict is more complex and trends about 5% higher; comparing the two is a good sanity check. The Katch-McArdle formula uses lean body mass directly: BMR = 370 + (21.6 × LBM in kg). It's more accurate for athletic individuals where total body weight overstates metabolism, but requires a body composition measurement.

A worked example: a 35-year-old male, 80 kg, 180 cm. Mifflin-St Jeor: (10 × 80) + (6.25 × 180) − (5 × 35) + 5 = 800 + 1125 − 175 + 5 = 1755 kcal/day. Harris-Benedict gives roughly 1830 kcal — a 4% spread. Multiplying by activity factor (1.2 sedentary, 1.55 moderately active, 1.9 very active) yields TDEE: 2,107 to 3,335 kcal/day depending on activity. The same person at age 65 would calculate about 1605 kcal BMR — a ~150 kcal/day reduction primarily reflecting muscle loss with age.

Limitations: the formulas estimate population averages. Individual BMR can vary 200–300 kcal/day from the formula prediction at the same body composition. Genetics, thyroid function, prior dieting history, ambient temperature, and even gut microbiome shift the actual value. Indirect calorimetry (measuring oxygen consumption) is the only way to get a personal BMR; clinical measurement runs $100–300. For weight management, the formula is a starting point — adjust based on observed weight change over 2–4 weeks. If predicted maintenance calories don't maintain weight, the formula underestimated or overestimated by some amount specific to you.