Lean Body Mass (LBM) is everything in your body that isn't fat: muscles, bones, organs, blood, water, and connective tissue.
Estimation Formulas
- Boer (men): LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
- Boer (women): LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
Why It Matters
LBM drives your basal metabolic rate — more lean mass = more calories burned at rest. Tracking LBM during weight loss ensures you're losing fat, not muscle.
LBM Changes With Age
Adults naturally lose 3–8% of lean body mass per decade after age 30 (a process called sarcopenia), accelerating after 60 without resistance training. This is a major driver of the metabolic slowdown often blamed on aging — much of it is actually attributable to muscle loss, which is partially preventable with strength training and adequate protein.
LBM vs. Fat-Free Mass
The terms are often used interchangeably, but technically fat-free mass excludes only pure fat, while lean body mass sometimes excludes essential fat too (the small amount needed for hormonal and organ function). In practice, most calculators and formulas treat them as equivalent.