What Is Basal Metabolic Rate?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — lying still, awake, in a thermoneutral environment, 12–14 hours after eating. It represents the energy required to keep your heart beating, lungs breathing, kidneys filtering, brain functioning, and body temperature stable.
BMR accounts for 60–75% of daily calorie expenditure for most sedentary people and 40–60% for very active individuals. The remaining calories come from digesting food (8–10%), unconscious movement like fidgeting and posture (15–30%), and intentional exercise (variable).
BMR is not the same as RMR (Resting Metabolic Rate). RMR is measured after only 8 hours of fasting and minimal physical rest, making it slightly higher than true BMR (typically by 10–20%). Most calculators — including ours — actually calculate RMR, which is more practical since true BMR requires hospital-level measurement conditions.
Mifflin-St Jeor vs. Harris-Benedict: Which Is More Accurate?
Two equations dominate BMR calculation. Harris-Benedict (1919) was the standard for 70 years: Men: BMR = 88.36 + (13.4 × kg) + (4.8 × cm) − (5.68 × age). Women: BMR = 447.6 + (9.25 × kg) + (3.10 × cm) − (4.33 × age).
Mifflin-St Jeor (1990) updated this with modern population data: Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5. Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161.
A 2005 study in the Journal of the Academy of Nutrition and Dietetics compared both equations against measured BMR and found Mifflin-St Jeor was more accurate for 82% of participants. The error was within 10% for most — acceptable for practical use. The American Dietetic Association now recommends Mifflin-St Jeor as the preferred equation.
Both equations are less accurate for very muscular individuals (underestimating BMR) and very obese individuals (overestimating). For these populations, the Katch-McArdle formula (which uses body fat percentage rather than overall weight) is more accurate: BMR = 370 + (21.6 × lean body mass in kg).