Free BMR Calculator — Basal Metabolic Rate

Basal Metabolic Rate is the number of calories your body burns in a state of complete rest — no digestion, no movement, no stress. It represents the minimum energy cost of staying alive: keeping your heart beating, your lungs breathing, your kidneys filtering, and your cells repairing themselves around the clock. For most adults, BMR accounts for 60–75% of total daily calorie expenditure, making it the single largest component of how many calories you burn each day.

Understanding your BMR gives you the most important number in any calorie plan. Every calorie target — whether for weight loss, maintenance, or muscle gain — is calculated relative to your BMR. Without knowing your metabolic floor, calorie goals are guesswork.

Two equations dominate BMR estimation: Harris-Benedict (1919, revised 1984) and Mifflin-St Jeor (1990). Both use the same four inputs — weight, height, age, and sex — but they differ in the coefficients applied and in how well they match measured resting energy expenditure in modern populations.

The original Harris-Benedict equation was developed in the early 20th century on a relatively small sample of subjects. Its 1984 revision by Roza and Shizgal corrected for known biases, improving accuracy. The Mifflin-St Jeor equation was developed thirty years later on a more representative sample, and head-to-head validation studies consistently find it comes closest to indirect calorimetry measurements — the laboratory gold standard for measuring actual BMR.

The formulas themselves look like this. Mifflin-St Jeor for men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women: the same formula but ending in −161 rather than +5. The difference between the male and female equations reflects average differences in lean body mass between sexes. This calculator uses Mifflin-St Jeor for all BMR estimates.

Neither formula is perfect. Both assume average body composition for a given height and weight. Someone with unusually high muscle mass — a competitive bodybuilder, for example — will have a higher true BMR than either equation predicts, because muscle tissue is metabolically active even at rest. Someone with very low muscle mass will have a lower BMR. Use these estimates as a well-informed starting point, then refine based on real-world results over two to four weeks.

Your BMR is the floor, not the ceiling. It tells you how many calories your body demands simply to stay alive — and it's never zero. Even sleeping, your metabolism is burning fuel to maintain body temperature, repair tissues, and keep organs functioning. You should never eat below your BMR for extended periods without medical supervision. Doing so forces the body into adaptive thermogenesis, where it downregulates metabolic rate to conserve energy — making future weight loss progressively harder.

BMR does not tell you how many calories you need each day for your actual lifestyle. That requires multiplying by an activity factor to get your TDEE. A sedentary office worker multiplies BMR by 1.2; someone with a physically demanding job and daily workouts may multiply by 1.9 or more. The gap between BMR and TDEE is significant: for a moderately active person, TDEE is typically 55% higher than BMR.

BMR also changes over time. It declines gradually with age as lean muscle mass naturally decreases — roughly 1–2% per decade after age 30. This is one reason why people often find it progressively harder to maintain a healthy weight as they age while eating the same amount. Resistance training is the most effective tool for slowing this decline, because building muscle raises BMR even at rest.