BMR Calculator

What your BMR actually pays for

Basal metabolic rate is the electricity bill of being alive: the energy spent keeping your heart beating, lungs breathing, brain thinking, kidneys filtering, and cells rebuilding — before you take a single step. For most adults it is the largest slice of daily energy use, typically 60–70%. Exercise, for all the attention it gets, is usually a far smaller line item. That proportion is why two people of different sizes can eat the same diet with opposite results, and why “fast” and “slow” metabolisms are mostly just bigger and smaller bodies with more or less lean tissue.

Reading the two equations side by side

For a 40-year-old woman, 165 cm and 68 kg, the calculator returns a Mifflin-St Jeor BMR of about 1,350 kcal and a revised Harris-Benedict value of about 1,414 kcal. The 64-kcal gap is not an error — it is two research teams fitting curves to different groups of people, in 1990 and 1984 respectively. Mifflin-St Jeor validates better against measured resting energy in modern adults, which is why it is the headline number here and the equation behind our calorie calculator. When the two equations agree within about 5%, as they do for most inputs, you can treat their average as a sensible working figure. When they diverge sharply — very high or very low body weights — both are extrapolating beyond their study data, and real measurement (indirect calorimetry) is the only way to know.

What moves the number — and by how much

• Weight: 10 kcal per kg. Each kilogram of body mass adds 10 kcal to resting burn in Mifflin-St Jeor. Losing 5 kg lowers BMR by ~50 kcal/day — one reason weight loss slows as it proceeds.
• Height: 6.25 kcal per cm. Taller bodies have more surface area and more tissue to run.
• Age: −5 kcal per year. Our example at 30 would read 1,400 kcal and at 50, 1,300 kcal — a steady 50 kcal per decade at the same size. Much of the real decline is muscle loss, not aging itself, which is why the trend is partly negotiable.
• Sex: a 166-kcal constant. The equations offset men and women by a fixed amount, standing in for average differences in lean mass. A muscular woman and a sedentary man of the same size can easily swap places in reality.

The mistake the activity table prevents

The most common misuse of a BMR number is eating at it. BMR is a floor, not a target: even a bedridden day burns more than basal, because digestion alone adds roughly 10% on top of everything else. Eating at BMR therefore creates an uncontrolled, usually aggressive deficit — our example would be under-eating by 270 kcal even on fully sedentary days (1,620 vs 1,350), and by more than 740 kcal on moderately active ones. The table above scales your BMR through the standard activity factors (1.2–1.9) precisely so you can see the range your real days fall into. Pick the level that matches your week honestly and treat that number as maintenance; deficits and surpluses should be measured from it, not from BMR.

Can you change your BMR?

Modestly, and in both directions. Resistance training adds muscle, and each kilogram of muscle burns roughly 10–13 kcal a day at rest — small per kilo, meaningful over years. In the other direction, prolonged severe dieting triggers adaptive thermogenesis: the body lowers resting burn beyond what the lost weight alone predicts, which is one reason crash diets rebound. Protein intake and sleep both help preserve lean mass while losing weight. If your goal is a target weight rather than a target burn, the BMI calculator frames a healthy range for your height, and the body fat calculator tracks whether the weight you keep is the kind that burns.