Lean body mass is everything that isn't fat — muscle, bone, organs, water. It drives metabolism, strength, drug dosing, and survival outcomes after illness.

Boer (1984) and Hume (1966) are the most widely cited modern formulas. James (1976) tends to underestimate at high BMIs, while Peters uses body surface area and is preferred in paediatrics.

If you have a measured body fat %, the direct calculation (Weight × (1 − BF%)) is the most accurate.