Fat-free mass is everything in your body except fat: muscle, organs, bone, water and glycogen. It is the basis for FFMI, for the protein range during a cut and for the Katch-McArdle formula for basal metabolic rate.
Formula
Fat-free mass (kg) = body weight (kg) × (1 - body fat (%) ÷ 100)
Example: At 75 kg and 12 percent body fat, that is 75 × 0.88 = 66.0 kg of fat-free mass. Using Katch-McArdle, this gives a basal metabolic rate of 370 + 21.6 × 66 = about 1,796 kcal. This is a worked example.
Not the same as muscle
Fat-free mass is not the same as muscle mass. Water and glycogen count too, so it can rise or fall within a few days, for example after eating more carbs or starting creatine. Short-term changes therefore say little about muscle growth.
What studies show
How much of a weight change is fat-free mass depends on your starting body fat. In an analysis of overfeeding studies (Forbes 2000), lean people gained about 60 to 70 percent of the added weight as fat-free mass, people with obesity only about 30 to 40 percent. When losing weight, the reverse applies: the leaner you are, the greater the risk of losing fat-free mass during a cut. More protein combined with strength training helps you keep it in a deficit.