CalcBMR Calculator (Mifflin-St Jeor)
Calculate basal metabolic rate using the Mifflin-St Jeor equation.
The Mifflin-St Jeor equation is considered the most accurate formula for estimating basal metabolic rate (BMR) in healthy individuals, outperforming older equations like Harris-Benedict in clinical validation studies. Your BMR represents the minimum calories your body needs to sustain vital functions at complete rest, accounting for 60–75% of your total daily energy expenditure.
Mifflin-St Jeor BMR Formula
Men: BMR = (10 × Weight) + (6.25 × Height) − (5 × Age) + 5
Women: BMR = (10 × Weight) + (6.25 × Height) − (5 × Age) − 161
Weight is in kilograms, height in centimeters, and age in years. The only difference between the male and female formulas is the constant term (+5 for men, −161 for women), reflecting the generally lower muscle mass and higher body fat percentage in women. For example, a 35-year-old woman weighing 68 kg at 165 cm tall: BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161 = 680 + 1031.25 − 175 − 161 = 1,375 kcal/day.
Worked Examples
Example 1: Middle-Aged Man
A 50-year-old man weighs 92 kg and is 180 cm tall.
BMR = (10 × 92) + (6.25 × 180) − (5 × 50) + 5 = 920 + 1125 − 250 + 5 = 1,800 kcal/day
His BMR of 1,800 calories represents the energy his body needs at complete rest for breathing, circulation, cell production, and neural function. With a sedentary lifestyle (activity factor 1.2), his TDEE would be 1,800 × 1.2 = 2,160 kcal/day. This means even without any exercise, his body burns 1,800 calories just to maintain basic physiological functions.
Example 2: Young Woman
A 22-year-old woman weighs 55 kg and is 160 cm tall.
BMR = (10 × 55) + (6.25 × 160) − (5 × 22) − 161 = 550 + 1000 − 110 − 161 = 1,279 kcal/day
Her BMR of 1,279 calories is lower than the man’s due to smaller body size and the female constant. With moderate activity (factor 1.55), her TDEE = 1,279 × 1.55 = 1,982 kcal/day. For healthy weight loss, she should not consume below her BMR of 1,279 kcal without medical supervision, as this could compromise essential bodily functions.
Common Uses
- Calculating the base metabolic rate as the foundation for TDEE and personalized calorie targets
- Clinical nutrition assessment for hospitalized patients requiring enteral or parenteral nutrition support
- Setting minimum calorie thresholds for safe weight loss diets to prevent metabolic adaptation
- Comparing metabolic rates across populations in epidemiological and nutritional research studies
- Evaluating metabolic health by comparing measured resting metabolic rate (via indirect calorimetry) to predicted BMR
- Designing refeeding protocols for individuals recovering from eating disorders or prolonged malnutrition
Common Mistakes
- Confusing BMR with TDEE and eating at BMR level while being physically active, creating an unintentionally large calorie deficit
- Using pounds and inches instead of kilograms and centimeters, which produces completely incorrect BMR values
- Assuming BMR is fixed and unchangeable, when increasing muscle mass through resistance training can raise BMR by 50–100 kcal/day
- Applying the Mifflin-St Jeor equation to obese individuals without adjustment, as it may overestimate BMR in those with high body fat percentages
Pro Tip
While BMR formulas provide useful estimates, indirect calorimetry (measuring oxygen consumption and carbon dioxide production) is the gold standard for determining actual resting metabolic rate. Devices like the FitMate or BodyGem cost $5,000–$15,000 but are available at many sports medicine clinics. If you cannot access indirect calorimetry, use the Mifflin-St Jeor equation and then validate it against real-world data: track your weight and calorie intake for 3–4 weeks. If weight is stable, your actual TDEE equals your average intake. Subtract the activity factor contribution to back-calculate your true BMR.
Frequently Asked Questions
The Mifflin-St Jeor equation was developed in 1990 using a more diverse and contemporary sample than the 1919 Harris-Benedict equation. Studies show Mifflin-St Jeor predicts BMR within 10% of measured values in 82% of individuals, compared to 67% for Harris-Benedict. It is now the standard equation recommended by the American Dietetic Association.
Yes. Each kilogram of muscle mass burns approximately 13 kcal/day at rest, compared to 4.5 kcal/day for fat tissue. Gaining 5 kg of muscle through resistance training could increase your BMR by approximately 65 kcal/day. Additionally, adequate protein intake, quality sleep, and avoiding prolonged severe calorie deficits help maintain a healthy metabolic rate.
Yes. Metabolic adaptation occurs during caloric restriction as the body becomes more efficient, reducing BMR by 5–15% beyond what would be expected from weight loss alone. This “starvation response” is why weight loss plateaus occur. Diet breaks (1–2 weeks at maintenance calories) and resistance training can help mitigate this adaptation.
BMR and RMR are similar but measured under different conditions. BMR requires strict conditions: measured after 8 hours of sleep and 12 hours of fasting in a thermoneutral environment. RMR is measured under less restrictive conditions and is typically 10–20% higher than BMR. For practical purposes, the terms are often used interchangeably in nutrition planning.