Lean body massLBM · Fat-free mass

Lean body mass (LBM) is your total weight minus your fat mass: muscle, bone, organs, connective tissue and body water. It's the counterpart to body-fat percentage.

It matters because lean mass — especially muscle — drives strength, metabolic rate and function, and preserving it is a key goal when dieting. Many nutrition and dosing calculations are based on lean mass rather than total weight, since fat is less metabolically demanding.

In practice, LBM is estimated from body-fat measurements, and the aim during fat loss is to lose fat while holding on to lean mass through protein and resistance training.