What BMR actually measures
Basal metabolic rate is the energy your body uses to keep you alive at complete rest — heart beating, lungs breathing, brain firing, cells repairing. It accounts for roughly 60–70% of total daily calorie burn for most adults. The bigger your body and the more muscle mass you carry, the higher your BMR; age, sex, and body composition explain most of the variation between people.
BMR vs TDEE — don't confuse them
BMR is what you'd burn in a coma. TDEE — Total Daily Energy Expenditure — is what you actually burn living your life: BMR plus digestion plus walking plus working plus training. TDEE is almost always 1.2× to 1.9× higher than BMR. Diet at BMR and you're guaranteed to feel terrible; diet at 15% below TDEE and the loss is sustainable.
The Mifflin-St Jeor equation
This calculator uses the Mifflin-St Jeor equation, the most accurate predictive formula for the general adult population (mean error around 5% vs. indirect calorimetry). It was validated against direct measurements in 1990 and remains the standard used by the Academy of Nutrition and Dietetics.
For men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
For women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Why BMR drops as you lose weight
A smaller body costs less to run. Lose 20 lb and your BMR can drop 100–200 calories — which is why diets stall and why recalculating every 10–15 lb matters. Strength training preserves lean mass, which preserves BMR, which preserves the size of the deficit you can run without misery.