What is lean body mass?
Lean body mass (LBM) is your weight minus body fat: muscle, bones, organs and water. It drives your metabolism, so tracking it matters more than the number on the scale when you diet or train.
This calculator uses three formulas — Boer, James and Hume — and reports the average, along with the fat mass and body fat percentage that result.
How it is calculated
Boer (men): 0.407 × kg + 0.267 × cm − 19.2
James (men): 1.1 × kg − 128 × (kg ÷ cm)²
Hume (men): 0.3281 × kg + 0.33929 × cm − 29.5336
Women use different constants for each formula. Body fat % = (weight − LBM) ÷ weight.
Worked example
Man, 70 kg, 170 cm
- Boer
- 54.7 kg
- James
- 55.3 kg
- Hume
- 51.1 kg
The average is about 53.7 kg, which means roughly 23% body fat.
Why LBM matters
- Muscle burns more calories than fat, so more LBM means a higher metabolism.
- Protecting LBM while dieting keeps you strong and prevents the weight coming back.
- Drug doses and calorie plans are often based on LBM, not total weight.