What it means
To estimate it, subtract fat mass from total weight. If you weigh 200 lb at 35% body fat, your fat mass is about 70 lb and your lean mass is about 130 lb.
Some weight lost during any diet comes from lean mass. In body-scan substudies of major GLP-1 trials, a meaningful share of weight lost was lean mass, roughly a quarter to around two-fifths depending on the study. Researchers are still debating how much of that is muscle versus water and organ size.
Resistance training and enough protein are the two habits best supported for holding on to muscle while losing weight.
Lean mass is also where most of your body water lives, so short-term scale swings after a salty meal or a hard workout often come from this part of your weight, not fat.
Why it matters
Lean mass drives your BMR and strength, so protecting it is a core goal of the protein-floor tool.