BMR is the number of calories your body burns at complete rest — the foundation of every calorie target. Here's how to calculate it using two different formulas and what to do with the number.
Before you can calculate how many calories to eat for any goal, you need your BMR — the baseline number of calories your body burns just to stay alive, without any activity at all.
Basal Metabolic Rate is the energy used by your body at complete rest to maintain basic physiological functions: breathing, circulation, cell production, temperature regulation, and organ function. It doesn't include any physical activity — that's added separately via an activity multiplier to get TDEE (Total Daily Energy Expenditure).
BMR accounts for roughly 60–75% of total daily calorie expenditure for most people — it's by far the biggest component of your energy budget.
The Mifflin-St Jeor equation, developed in 1990, is the most widely validated for predicting BMR in the general population:
The Mifflin-St Jeor formula tends to be about 5% more accurate on average, particularly for people who are overweight. Both are estimates — individual variation of ±10% is common.
Age: BMR decreases approximately 1–2% per decade from age 20. This is partly due to muscle loss (sarcopenia) — not an inevitable decline but correlated with activity levels.
Muscle mass: Muscle tissue burns roughly 6 calories per pound per day at rest; fat tissue burns about 2. Every kilogram of muscle added raises BMR by roughly 13–15 calories/day. Building muscle is one of the few ways to actively raise your BMR.
Thyroid function: The thyroid regulates metabolic rate. Hypothyroidism meaningfully lowers BMR; hyperthyroidism raises it. Unexplained weight changes warrant thyroid testing.
Body size: Larger bodies have higher BMRs, all else equal. This is why BMR drops when you lose weight — a well-known challenge in long-term weight management.