BMR Calculator (Mifflin, Harris-Benedict, Katch-McArdle)

What BMR is

Basal Metabolic Rate (BMR) is the calories your body burns at complete rest — the energy required to keep your heart beating, lungs breathing, brain thinking, kidneys filtering, and so on. It's the floor under which you can't reduce calorie intake without harming basic function.

BMR is roughly 60-75% of your total daily energy expenditure (TDEE). Adding activity, digestion (the thermic effect of food), and exercise gives TDEE — the number you actually need to maintain weight.

Why three formulas

Mifflin-St Jeor (1990) is the modern default. Multiple validation studies show it's the most accurate of the simple BMR formulas — within 10% for most healthy adults. Use this unless you have a specific reason not to.

Harris-Benedict (1919, revised 1984) is the older, well-known formula. It tends to overestimate BMR by 5-10% in modern populations because the original sample didn't reflect today's body compositions. Useful when comparing to historical literature.

Katch-McArdle uses lean body mass instead of total weight. If you have a body fat percentage measurement (DEXA, BodPod, or even a decent body composition scale), this is the most accurate for muscular individuals — Mifflin and Harris-Benedict overestimate fat-mass contribution to BMR.

How to use the result

Pure BMR is rarely your goal — you want TDEE. Multiply BMR by an activity factor: 1.2 (sedentary), 1.375 (light), 1.55 (moderate), 1.725 (active), 1.9 (very active). For a 1500 kcal BMR with moderate activity, TDEE is about 2325 kcal.

For weight loss, eat 250-500 kcal below TDEE per day (0.25-0.5 kg/week loss). For weight gain, eat 250-500 above. Don't go below BMR for sustained periods — you'll lose lean mass and your metabolism will adapt downward.

Re-check BMR every 4-6 kg of weight change. Lighter bodies need fewer calories at rest; this is why diets stall as you progress.