BMR Calculator

What BMR actually measures

Basal metabolic rate is the energy your body uses at complete rest to run essential functions: breathing, circulation, cell repair and brain activity. It accounts for roughly 60 to 70 percent of daily energy use for most people, which is why it dominates any calculation of total needs. Strictly, BMR is measured after an overnight fast in a thermally neutral room at complete rest; what a calculator gives you is an estimate of that.

Which equation

Mifflin-St Jeor is the current standard and generally the most accurate for the general population. Harris-Benedict is older and tends to overestimate slightly. Katch-McArdle uses lean body mass rather than total weight and is more accurate for anyone lean or muscular, but requires a body fat percentage, which most people do not have measured reliably. If you have a genuine DEXA figure, use Katch-McArdle; otherwise Mifflin-St Jeor.

Activity multipliers are the weak link

BMR is multiplied by an activity factor to give total daily energy expenditure, and this is where most of the error enters. The factors are coarse bands, people consistently overestimate their own activity level, and the same nominal level covers a wide real range. A sedentary office worker who walks a lot is not the same as one who does not. Treat the resulting TDEE as a starting hypothesis, not a measurement.

Individual variation is large

Two people matched on age, sex, height and weight can differ in actual BMR by 200 to 300 calories a day, driven by body composition, thyroid function, genetics and non-exercise activity thermogenesis, which is the energy spent fidgeting and moving unconsciously and which varies enormously between individuals. Any equation gives a population average, not your number.

Use it as a starting point and adjust

The reliable method is to take the estimate, eat at that level consistently for two to three weeks while tracking weight, and adjust based on what actually happens. Your own trend data beats any equation, because it accounts for everything the equation cannot see. Weigh at the same time daily and use a weekly average, since day-to-day fluctuation is mostly water.

This is general information, not medical or dietary advice. Anyone with a metabolic condition, an eating disorder history, or who is pregnant should work with a clinician or dietitian.