What is BMR Calculator?
Basal metabolic rate is the energy your body uses to stay alive at complete rest: breathing, circulating blood, maintaining temperature and repairing tissue. It accounts for roughly 60–70% of the calories you burn in a day, which is why it anchors every calorie target. BMR depends mainly on body size and composition — more muscle raises it, and it falls with age and as you lose weight. This calculator returns BMR from three published equations so you can see the range: Mifflin-St Jeor (the current default), Harris-Benedict (the older revised standard) and Katch-McArdle (which uses lean body mass). Multiply BMR by an activity factor to get the calories you actually need per day.
Common Uses for BMR Calculator
- Find the resting calorie floor for your body
- Compare BMR formulas to see the estimation range
- Build a calorie target from first principles
- Track how BMR falls as you lose weight
What goes into BMR
Roughly 60% of resting energy goes to organs, with the brain, liver, heart and kidneys unusually demanding per gram. Muscle is comparatively cheap at rest — about 13 kcal per kg per day — which is why 'muscle burns a lot of calories' is overstated, though it still helps.
Why BMR falls with dieting
A smaller body has a lower BMR, and the metabolism also adapts downward during prolonged restriction — the 'adaptive thermogenesis' effect. Both are reasons weight loss slows over time and why periodic maintenance breaks help.
From BMR to a real target
Multiply BMR by an activity factor of 1.2 (sedentary) to 1.9 (very active) to get TDEE, then adjust for your goal. That two-step is the whole basis of evidence-based calorie planning.