Katch Mcardle Calculator
Compute your BMR the body-composition way. Enter weight (lb or kg) and body fat percent; the Katch-McArdle formula converts your lean mass into resting calories, and an activity multiplier turns that into a full-day TDEE estimate.
Example: with Weight 180 · Weight unit lb (pounds) · Body fat (%) 20 · Activity level Lightly active — 1-3 days/week (×1.375) → BMR (Katch-McArdle): 1,781 kcal/day.
- Lean body mass144.0 lb (65.3 kg)
- Estimated TDEE2,449 kcal/day at ×1.375
Computed by the calculator below using its default values. Change any input to see your own numbers.
BMR = 370 + 21.6 × lean body mass (kg). Because it keys on lean tissue, it fits muscular and very lean people better than weight-only equations.
Why key BMR to lean mass
Muscle, organs, and other lean tissue do nearly all of your resting calorie burning; fat tissue idles at a fraction of the rate. Weight-only equations like Mifflin-St Jeor or Harris-Benedict bake in an average body composition, so they overestimate for people carrying extra fat and underestimate for the unusually muscular. Katch-McArdle skips the averaging: 370 kcal baseline plus 21.6 kcal for every kilogram of lean mass.
The trade is that you must know your body fat percent. If you have a decent estimate — calipers, a Navy-formula tape measurement, or a DEXA scan — Katch-McArdle is arguably the best simple BMR formula for lean or muscular bodies. If you would be guessing, a weight-based equation with no guess in it often lands closer.
How it’s calculated
Lean body mass = weight × (1 − body fat % ÷ 100), in kg (1 lb = 0.45359237 kg). BMR = 370 + 21.6 × LBM (Katch-McArdle). TDEE = BMR × activity multiplier (1.2 sedentary through 1.9 extra active — the standard Harris-Benedict-style factors). Example: 180 lb at 20% fat = 65.3 kg lean, BMR ≈ 1,781 kcal.
The formula is only as good as your body-fat number (±3% error moves BMR ±50-60 kcal), and activity multipliers are rough population factors — treat results as planning estimates, not medical advice, and involve a dietitian for clinical calorie prescriptions.
Katch-McArdle BMR at 180 lb across body-fat levels
| Body fat | Lean mass | BMR |
|---|---|---|
| 10% | 162.0 lb (73.5 kg) | ≈ 1,957 kcal |
| 15% | 153.0 lb (69.4 kg) | ≈ 1,869 kcal |
| 20% | 144.0 lb (65.3 kg) | ≈ 1,781 kcal |
| 25% | 135.0 lb (61.2 kg) | ≈ 1,693 kcal |
| 30% | 126.0 lb (57.2 kg) | ≈ 1,605 kcal |
Computed with BMR = 370 + 21.6 × LBM(kg) for a 180 lb adult; rounded to the nearest kcal.
Common mistakes
- Guessing body fat percent — every point of error moves BMR about 18 kcal at 180 lb, and guesses commonly miss by 5+.
- Comparing your Katch-McArdle BMR against a tracker's TDEE — one is resting-only, the other includes all movement.
- Using an aggressive activity multiplier and also logging workouts as extra burn — that counts training twice.
- Expecting precision to the calorie: all BMR equations carry roughly ±10% individual spread.
Frequently asked questions
What is the Katch-McArdle formula?
BMR = 370 + 21.6 × lean body mass in kilograms, where lean mass = weight × (1 − body fat%). A 180 lb person at 20% body fat has 65.3 kg of lean mass, so BMR = 370 + 21.6 × 65.3 ≈ 1,781 kcal/day.
How is it different from Mifflin-St Jeor or Harris-Benedict?
Those use weight, height, age, and sex, implicitly assuming average body composition. Katch-McArdle uses lean mass directly — sex and age drop out because they mostly acted as proxies for composition. It shines for muscular or very lean people.
Do I need an exact body fat percentage?
The closer the better: a 3-point error shifts BMR by roughly 50-60 kcal. Tape-measure (Navy) or caliper estimates are fine for planning; if you only have a wild guess, a weight-based formula may actually be more reliable.
Is Katch-McArdle more accurate for women?
It has no separate female equation because lean mass already carries the difference. For women with a solid body-fat estimate it performs well; with a guessed one, Mifflin-St Jeor's explicit female constant is the safer bet.
Can I eat at my BMR to lose weight?
BMR is your floor, not a target — you burn more than it every day you move at all. Set intake relative to TDEE instead, and involve a doctor or registered dietitian before sustained intakes near or below BMR.