BMR Calculator — Basal Metabolic Rate

Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to sustain essential biological functions — including breathing, circulation, cell repair, temperature regulation, hormone production, and neurological activity. BMR represents the minimum caloric energy your body requires to keep you alive if you were completely at rest for 24 hours, doing nothing beyond existing.

BMR typically accounts for 60–75% of total daily energy expenditure in sedentary individuals, making it by far the largest component of your daily calorie burn. The remaining calories come from physical activity (15–30%) and the thermic effect of food — the energy used to digest and metabolize what you eat (8–10%). Understanding your BMR is therefore the foundation of any evidence-based approach to calorie management, weight loss, or muscle gain.

This calculator uses the Mifflin-St Jeor equation, published in 1990 and validated across large, diverse adult populations. The Academy of Nutrition and Dietetics, American Dietetic Association, and most clinical nutrition guidelines recommend it as the most accurate predictive equation for BMR in non-obese adults, with a mean error of approximately 10% compared to gold-standard indirect calorimetry measurements.

Example 1 — Woman, 28 years old, 65 kg, 168 cm: BMR = (10×65) + (6.25×168) − (5×28) − 161 = 650 + 1,050 − 140 − 161 = 1,399 kcal/day Moderately active TDEE: 1,399 × 1.55 = 2,168 kcal/day To lose 0.5 kg/week: target ~1,668 kcal/day (500 kcal deficit below TDEE)

Example 2 — Man, 40 years old, 85 kg, 178 cm: BMR = (10×85) + (6.25×178) − (5×40) + 5 = 850 + 1,112.5 − 200 + 5 = 1,767.5 kcal/day Lightly active TDEE: 1,768 × 1.375 = 2,431 kcal/day To maintain weight: eat approximately 2,431 kcal/day

Knowing your BMR is the single most important number in any rational, evidence-based approach to nutrition and body composition. Without understanding your individual baseline calorie needs, it is impossible to create a meaningful calorie deficit for fat loss or an appropriate surplus for muscle building. The most common reason popular diets fail is that they prescribe arbitrary calorie targets — such as '1,200 calories for all women' — with no relationship to individual metabolic requirements.

Clinical research consistently shows that eating significantly below BMR triggers a biological survival response called adaptive thermogenesis: the body reduces metabolic rate by 15–30% to conserve energy. This metabolic adaptation is the primary reason why aggressive crash diets produce initial weight loss but lead to weight regain — often with additional fat — once normal eating resumes. Studies published in the New England Journal of Medicine tracking Biggest Loser contestants found measurable metabolic adaptation persisting 6 years after the competition.

BMR also declines naturally with age — approximately 1–2% per decade after age 30 — largely due to progressive loss of lean muscle mass (sarcopenia). By age 70, a person who has been sedentary since their 30s may have a BMR 15–20% lower than in their youth, requiring significantly fewer calories to maintain weight. This is why maintaining muscle mass through regular resistance training is one of the most powerful strategies for long-term metabolic health and healthy body weight management.

The Mifflin-St Jeor equation provides a statistically derived estimate, not a direct measurement. The average prediction error is approximately 10%, meaning your actual BMR could be 10% higher or lower than the calculated value. For a person with a calculated BMR of 1,600 kcal/day, the real value could range from 1,440 to 1,760 kcal/day. Individual factors like genetics, hormonal status, medications, and body composition affect actual metabolic rate in ways the equation cannot capture.

This calculator uses height, weight, age, and sex as inputs — it does not directly measure lean body mass. For athletes and highly muscular individuals, the equation may underestimate BMR because it does not account for the extra metabolic activity of muscle tissue. In these cases, the Katch-McArdle formula — which uses lean body mass as input — tends to be more accurate. Conversely, in individuals with metabolic conditions such as hypothyroidism, Cushing's syndrome, or polycystic ovary syndrome (PCOS), actual BMR may be substantially lower than predicted.

BMR is also not a fixed number — it changes. Prolonged calorie restriction (crash dieting) decreases BMR through metabolic adaptation. Fever, pregnancy, hyperthyroidism, and post-surgical recovery increase it. For best results, recalculate your BMR every 4–6 weeks during active weight management, as changes in body weight shift your calorie requirements.