BMR Calculator

What is BMR (Basal Metabolic Rate)?

Basal Metabolic Rate (BMR) is the number of calories your body burns per day to sustain vital functions at complete rest — breathing, circulation, cell production, protein synthesis, and thermoregulation. It represents the minimum energy required to keep you alive and typically accounts for 60–75% of total daily calorie burn. For a sedentary person, BMR dominates energy expenditure; for a highly active athlete, it drops to 50–60% of daily calories. In a clinical setting, BMR is measured by indirect calorimetry — analyzing inhaled and exhaled gases — after 12 hours of fasting and 8 hours of sleep in a thermoneutral room. Because direct measurement is impractical outside research, predictive equations like Mifflin-St Jeor estimate BMR from weight, height, age, and sex, typically within 10% of measured values for healthy adults.

How to calculate BMR with the Mifflin-St Jeor equation

The Mifflin-St Jeor equation (1990) is the current standard for estimating BMR in adults. For men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. For women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. A 30-year-old man weighing 80 kg at 180 cm tall has a BMR of (10 × 80) + (6.25 × 180) − (5 × 30) + 5 = 1780 kcal/day. A 30-year-old woman weighing 65 kg at 168 cm has (10 × 65) + (6.25 × 168) − (5 × 30) − 161 = 1389 kcal/day. The Academy of Nutrition and Dietetics recommends Mifflin-St Jeor over older equations because it was validated on modern body compositions and predicts measured BMR within ±10% for approximately 82% of non-obese individuals, per a 2005 meta-analysis by Frankenfield et al. in the Journal of the American Dietetic Association.

BMR vs TDEE — what's the difference?

BMR is calories burned at complete rest; TDEE (Total Daily Energy Expenditure) is calories burned across an entire day including all movement. TDEE equals BMR multiplied by an activity factor ranging from 1.2 (sedentary) to 1.9 (very heavy exercise plus physical job). For a 1780 kcal BMR and a moderately active lifestyle (factor 1.55), TDEE is approximately 2760 kcal. TDEE breaks down into three additional components beyond BMR: the thermic effect of food (TEF, about 10% of calories eaten), exercise activity thermogenesis, and non-exercise activity thermogenesis (NEAT — fidgeting, standing, walking). NEAT can vary by up to 2000 kcal/day between individuals with otherwise similar habits, which is why two people with identical BMR and workouts can have very different weight trajectories. For meal planning and weight management, use TDEE, not BMR.

Why does BMR decline with age?

BMR declines roughly 1–2% per decade in adulthood, primarily due to the loss of lean muscle mass (sarcopenia) rather than an intrinsic metabolic change. A landmark 2021 Science paper by Pontzer et al. analyzed data from 6,421 people across 29 countries using doubly-labeled water measurements and found that adjusted metabolic rate remains stable between ages 20 and 60, then declines about 0.7% per year after age 60. The age-related decline most people experience between 30 and 60 is better explained by reduced muscle mass and lower physical activity than by metabolic aging itself. Resistance training preserves lean mass and therefore BMR — a 2020 meta-analysis in Sports Medicine found strength training twice weekly could raise resting metabolic rate by 5–9% in older adults. Protein intake above 1.6 g/kg bodyweight supports lean-mass retention with age.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

Three predictive equations dominate BMR estimation. Mifflin-St Jeor (1990) is the default recommendation for healthy adults. Harris-Benedict, published in 1919 and revised in 1984, tends to overestimate BMR by 5–10% in sedentary individuals but remains in common use. Katch-McArdle uses lean body mass instead of total body weight, which makes it more accurate for very muscular or very lean individuals — but it requires knowing your body fat percentage. For most people, Mifflin-St Jeor produces the best estimate. If you are a lifter with a DEXA scan or reliable body-fat reading, Katch-McArdle is a reasonable alternative. Harris-Benedict is mostly kept around for continuity in older clinical references and should not be your first choice for new calculations.