BMR Calculator (Mifflin, Harris-Benedict, Katch-McArdle)
Enter age, sex, height, weight, and optionally body-fat % to compute BMR with three different clinical formulas. Useful for diet planning and tracking metabolic baseline.
How it works
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.
Frequently asked questions
›Which formula should I use?
Mifflin-St Jeor for general use. Katch-McArdle if you have a body fat measurement and lean body composition matters (athletes, bodybuilders).
›Why does Harris-Benedict give a different number?
It was developed in 1919 from a sample that didn't reflect modern body compositions. Most reviewers find it overestimates BMR by 5-10% in today's populations. Use Mifflin instead unless your reference material specifies Harris-Benedict.
›Can I trust my body fat % measurement?
DEXA and underwater weighing are gold standard. BodPod and skin calipers (with a trained measurer) are good. Bathroom-scale bioelectrical impedance is rough — accuracy varies day to day with hydration. Pick the most accurate measurement you have access to.
›Is BMR the same as resting metabolic rate (RMR)?
Very close, slightly different. BMR is measured under strict fasted, awake-but-resting conditions. RMR is less strict — usually after waking, no fasting required. RMR is typically 5-10% higher than BMR. The formulas estimate BMR specifically.
›How do I find my TDEE?
Multiply BMR by activity factor: 1.2 (no exercise) to 1.9 (athlete). Most office workers are 1.4-1.55. Use our calorie-calculator tool which combines BMR and activity for direct TDEE.
›Do I lose weight by eating below BMR?
You'll lose weight, but also lean mass and metabolic rate over time. Better: eat 250-500 below TDEE (not BMR). Sustainable cuts use TDEE deficits, not BMR floors.
›Why does my BMR drop with age?
Lean body mass declines with age (~3-5% per decade after 30 unless you actively maintain it with strength training). Mifflin's age coefficient (-5 per year) approximates this.
›Does the data leave my browser?
No. Calculation runs locally.
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