What FFMI measures
The Fat-Free Mass Index, or FFMI, expresses lean body mass relative to height, similar in form to BMI but using fat-free mass instead of total weight. It is used as a rough way to gauge how much muscle a person carries for their frame, independent of body fat.
Higher values reflect more lean mass per unit of height.
How the distribution is interpreted
Across the general male population, FFMI tends to cluster near the low twenties, with drug-free trained athletes in older analyses topping out around the mid-twenties. Values well beyond that range have historically been cited as a marker that raises the likelihood of anabolic drug use, though FFMI is an imperfect, population-level estimate rather than a definitive test.
Individual genetics, measurement error, and body fat all affect the number, so it should be read as a guide and not proof. This is general educational information, not a clinical or diagnostic tool.