BMR Calculator

BMR, or basal metabolic rate, is the energy your body spends doing nothing at all: breathing, circulating blood, maintaining body temperature, repairing cells. It is measured lying still, awake, after a 12-hour fast. For most adults BMR accounts for 60 to 75% of total daily calories, which makes it the single largest component of what you burn.

BMR is measured under strict laboratory conditions: complete rest, fasted, in a thermally neutral room. RMR (resting metabolic rate) is measured under looser conditions and comes out roughly 10% higher because it includes some digestion and minor movement. The terms are used interchangeably in everyday contexts, and every calculator you will find online, including this one, actually estimates something closer to RMR.

Mifflin-St Jeor for most people. It was validated against indirect calorimetry on a modern population and predicts within about 10% for roughly 80% of adults. Harris-Benedict, even in its 1984 revision, runs about 5% high because its 1919 study group was leaner and more active. Katch-McArdle beats both if you know your body fat percentage, because it works from lean body mass, the tissue that actually drives resting expenditure.

BMR is what you burn at complete rest. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor, so it adds movement, exercise and the energy cost of digesting food. TDEE is always higher: even a fully sedentary day comes to about 1.2 times BMR. Set calorie goals against TDEE, not BMR.

No. Eating at your BMR means eating as though you spent the entire day motionless, which creates a deficit of several hundred to a thousand calories before you have moved. That is aggressive enough to cost you muscle and, for many people, drops below the 1,200 (women) or 1,500 (men) calorie floor. Subtract 250 to 500 calories from TDEE instead.

Body size is the dominant input, so smaller and lighter people get lower numbers, and every formula subtracts for age. Body composition matters too: muscle burns roughly three times more at rest than fat per kilogram, so two people at the same weight can differ by 200 calories or more. If you have been in a long calorie deficit, adaptive thermogenesis can suppress actual BMR by 10 to 15% below the predicted figure.

Somewhat, and slowly. Adding muscle is the only durable lever: each kilogram of muscle adds roughly 13 calories a day at rest, so a serious year of resistance training might buy you 50 to 100 calories. That is real but modest. Increasing daily movement changes your TDEE far more than anything you can do to BMR, which is why step count moves the needle faster than metabolism hacking.