Estimate lean body mass (everything that is not fat). Use body fat % if you have it, or height and weight with the Boer formula.
Lean body mass (LBM) is your weight minus estimated fat mass. It includes muscle, bone, organs and water. Tracking LBM over time (with consistent body-fat estimates) helps you see whether weight changes are coming more from fat or from lean tissue.
Lean body mass (LBM) is your weight minus fat mass — muscle, bone, organs and water. From body fat %: LBM = weight × (1 − body fat%/100). The Boer formula estimates LBM from height and weight when fat % is unknown.
Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2. Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. This calculator runs both the body-fat method and the Boer method.
Scale weight mixes fat and lean tissue. Rising or stable LBM with falling fat mass means better body composition. That is a stronger signal of progress than weight alone during a cut or recomp.
Progressive strength training, adequate protein (often 1.6–2.2 g/kg), enough calories (surplus or at least maintenance), and sleep support muscle growth. Beginners can sometimes gain LBM even in a mild deficit.