What is lean body mass?
Lean body mass (LBM) is your total weight minus your fat: muscle, bone, organs, connective tissue and water. It's a useful number because your metabolism, strength and protein needs all scale with lean mass, not total weight. Two people at the same weight can have very different LBM.
How it's calculated
- From body fat % (most accurate): LBM = weight × (1 − body fat% ÷ 100).
- Boer, James & Hume formulas estimate LBM from height and weight when body fat is unknown. Boer is a solid default for most adults.
Why LBM matters
- Protein: targets are often set per kg of lean mass (roughly 1.6–2.2 g/kg of bodyweight, higher relative to LBM when lean).
- Metabolism: the Katch-McArdle BMR formula uses lean mass directly — see the TDEE calculator.
- Progress: when dieting, preserving LBM (not just losing weight) is the real goal.
LBM vs fat-free mass — same thing?
Almost. "Fat-free mass" excludes all fat; "lean body mass" technically includes a small amount of essential fat. In everyday use they're treated as the same.
Which formula should I trust?
If you know your body fat %, use that — it's the most personal. Otherwise Boer is a reliable general estimate; the three usually agree within a kilogram or two.
How do I raise my LBM?
Resistance training plus enough protein and calories builds muscle. Get your body fat with the body fat calculator to track lean gains over time.