How to Calculate Basal Metabolic Rate (BMR)
Learn how to calculate your Basal Metabolic Rate using the Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle equations. Understand what BMR measures and how to use it to set calorie goals.
What Is Basal Metabolic Rate?
Basal Metabolic Rate (BMR) is the minimum number of calories your body requires to sustain life while at complete rest — not sleeping, not digesting food, and in a thermoneutral environment. It accounts for the energy cost of maintaining organ function: the brain uses roughly 20% of resting energy, the liver approximately 27%, and skeletal muscle collectively about 18%. BMR typically represents 60–70% of total daily energy expenditure, making it the largest contributor to calorie needs by far. Knowing your BMR is the essential first step in calculating total calorie needs.
The Mifflin-St Jeor Equation
The Mifflin-St Jeor equation (1990) is the most accurate BMR formula for the general population and is recommended by the Academy of Nutrition and Dietetics. For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161. It has a mean accuracy within 10% of measured values in most study populations and outperforms older formulas like Harris-Benedict across a range of body types.
The Harris-Benedict Equation
The original Harris-Benedict equation was published in 1919 and revised in 1984. The revised (Roza & Shizgal) version is: For men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age). For women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age). While historically significant and widely used, it tends to overestimate BMR for obese individuals and is less accurate than Mifflin-St Jeor in most research comparisons. It remains in widespread clinical use due to familiarity.
The Katch-McArdle Equation
The Katch-McArdle formula is unique because it uses lean body mass (LBM) rather than total body weight, making it the most accurate equation for lean or athletic individuals: BMR = 370 + (21.6 × LBM in kg). Lean body mass = total weight − fat mass; for example, a 70 kg person with 15% body fat has LBM = 70 × (1 − 0.15) = 59.5 kg, giving BMR = 370 + (21.6 × 59.5) = 1,655 kcal. Using this formula requires a reliable body fat percentage measurement — from calipers, BIA, or DEXA. It is particularly valuable for athletes and individuals who have lost significant fat while gaining muscle.
Factors That Affect BMR
Body composition is the largest individual determinant of BMR: muscle tissue is about three times more metabolically active than fat tissue at rest. Age reduces BMR partly due to natural loss of muscle mass (sarcopenia), which declines at approximately 3–8% per decade after age 30 without resistance training. Sex differences exist because males generally have greater muscle mass and lower body fat percentage, resulting in higher absolute BMR values. Thyroid hormone is the primary hormonal regulator of BMR — hyperthyroidism can raise it by 50–100%, while hypothyroidism can reduce it by 20–40%.
From BMR to Total Calorie Needs
BMR alone underestimates total calorie requirements because it excludes the energy cost of digestion (thermic effect of food, ~10% of calories), physical activity, and non-exercise activity thermogenesis (NEAT — fidgeting, walking, posture). Multiply BMR by an activity factor to arrive at TDEE: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active, and 1.9 for extremely active. Calorie goals for weight loss or gain are then set by adjusting above or below the TDEE value. Recalculate BMR whenever body weight changes by more than 5 kg, as the formula inputs will shift meaningfully.
Measuring True BMR vs. Estimation
True BMR can only be measured under strict laboratory conditions: fasted for 12 hours, at physical and mental rest, in a thermoneutral environment, using indirect calorimetry (measuring oxygen consumption and CO₂ production). Indirect calorimetry is considered the clinical gold standard and is available in some hospitals and research facilities. Resting Metabolic Rate (RMR), measured in less strict conditions, is approximately 10–20% higher than true BMR and is the value most commercial metabolic tests report. For practical purposes, Mifflin-St Jeor provides an excellent estimate for the vast majority of users.
Try These Calculators
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Related Guides
How to Calculate Daily Calorie Needs
Find out how to calculate your Total Daily Energy Expenditure (TDEE) using BMR and activity level multipliers. Use this to set accurate calorie goals for weight loss, maintenance, or gain.
How to Calculate Macronutrients
Learn how to calculate your optimal protein, carbohydrate, and fat intake based on your calorie goals and health objectives. Understand macro ratios for weight loss, muscle gain, and general wellness.
How to Calculate Body Fat Percentage
Learn the main methods for calculating body fat percentage, including skinfold calipers, the U.S. Navy method, and DEXA scanning. Understand healthy ranges by age and sex.