BMR (Basal Metabolic Rate) Calculator

The Mifflin-St Jeor equation, developed in 1990, was validated against measured BMR using indirect calorimetry and found to be more accurate for modern populations. The original Harris-Benedict equation from 1919 tends to overestimate BMR by approximately 5%. A 2005 study in the Journal of the American Dietetic Association confirmed that Mifflin-St Jeor predicted measured BMR within 10% for more people than any other equation tested.

Yes, BMR decreases during caloric restriction through a process called adaptive thermogenesis—the body reduces energy expenditure beyond what's explained by lost mass. Research from the Biggest Loser study showed participants' metabolisms slowed significantly during rapid weight loss. To minimize this: lose weight gradually (0.5-1% body weight per week), maintain protein intake (1.6-2.2g/kg), and incorporate resistance training to preserve muscle mass.

The primary reason is body composition differences. Men generally have more lean muscle mass and less body fat than women at comparable weights. Muscle tissue is metabolically more active than fat tissue, requiring more energy at rest. Studies show that when adjusted for fat-free mass, the sex difference in BMR largely disappears. Hormonal differences (testosterone vs. estrogen) also influence muscle development and fat distribution patterns.

Prediction equations like Mifflin-St Jeor estimate BMR with approximately ±10% accuracy for most people. However, individual variation can be significant—research shows actual BMR can vary by up to 26% between people of similar age, sex, weight, and height. Factors not captured by equations include thyroid function, genetics, body composition, and metabolic history. For precise measurements, indirect calorimetry (measuring oxygen consumption) is the gold standard.

Muscle increases BMR, but the effect is often overstated. Research shows that 1 pound (0.45 kg) of muscle burns approximately 6 calories per day at rest, compared to 2 calories for fat. So gaining 10 pounds of muscle would increase BMR by only about 40-60 kcal/day. The real metabolic benefit of muscle comes during and after exercise (EPOC), improved insulin sensitivity, and overall physical capability—not dramatically elevated resting metabolism.

BMR decreases approximately 1-2% per decade after age 20. The main driver is sarcopenia—age-related muscle loss of 3-8% per decade after 30. Additionally, hormonal changes (declining growth hormone, testosterone, thyroid function) and reduced cellular metabolic activity contribute. However, this decline isn't inevitable: longitudinal studies show that maintaining physical activity and muscle mass through resistance training can substantially slow age-related BMR decline.