Lean body mass (LBM) is your total weight minus your fat mass. It includes muscle, bone, water, organs, and other non-fat tissues. LBM is important for calculating protein needs, setting calorie targets, and tracking muscle gain progress.

Fat-Free Mass Index (FFMI) normalizes lean mass for height, similar to how BMI normalizes weight. An FFMI of 18-20 is average for men, 20-22 is above average, and 22-25 indicates significant muscle development. Values above 25 are rare without performance-enhancing substances.

Building lean mass requires progressive resistance training, adequate protein intake (0.7-1.0g per pound of body weight), a slight caloric surplus, and sufficient sleep. Consistency over months and years produces the best results. Aim to gain 0.5-1 pound of lean mass per month.