Lean Body Mass Calculator
Everything you weigh that is not fat — muscle, bone, organs and water — by three published formulas.
Lean body mass is your total weight minus your fat mass. It is the number worth protecting while losing weight, the basis of sensible protein targets, and the input the Katch–McArdle metabolic equation asks for.
How this is worked out
Boer (1984) is the default and generally performs best for adults across a wide weight range. James (1976) and Hume (1966) are shown alongside it; all three predict lean mass from height and weight alone, which means they cannot tell a lifter from a sedentary person of the same size.
If you enter a measured body fat percentage, the calculator uses it directly — weight minus fat — and treats the three formulas as a sanity check rather than the answer.
This calculator runs entirely in your browser. Nothing you enter is transmitted to us, saved on our servers, or shared with anyone.
Common questions
What is the difference between lean body mass and muscle mass?
Lean body mass includes bone, organs, connective tissue and body water as well as muscle. Skeletal muscle is usually a bit under half of it.
Why does lean mass matter when losing weight?
Lean tissue burns more energy at rest than fat does, and losing it makes weight easier to regain. Enough protein and some resistance training are what protect it during a deficit.