What counts as lean body mass#
Lean body mass is everything that is not fat: muscle, bone, organs, skin, and water. It is not the same as muscle mass — muscle is the largest changeable part of it, but organs and bone contribute a fixed share. When your lean mass trends up over months of training while body fat holds steady, that trend is muscle growth.
Two ways to estimate it#
If you know your body fat percentage, lean mass is simple arithmetic: weight × (1 − body fat). That is the most accurate option here — measure it with the Navy tape method if you have a tape measure. Without a body fat number, the Boer, James, and Hume formulas estimate lean mass from height, weight, and sex. Boer is generally considered the most reliable of the three and is the one used in clinical settings; the spread between the formulas is a fair indication of the uncertainty involved.
What to do with the number#
- Set protein by lean mass: if you are significantly overweight, dosing protein per kilogram of lean mass (2.2–2.6 g/kg) is more sensible than per kilogram of total bodyweight.
- Track muscularity: the FFMI calculator indexes your lean mass to your height so you can compare it against trained and untrained norms.
- Judge a cut: re-estimate monthly while dieting. If weight is dropping but lean mass is holding, the deficit is working; if lean mass falls with it, eat more protein, slow the rate, or lift heavier.