FFMI answers a question BMI can't: how muscular are you for your height? By stripping out fat and normalising for stature, it gives a fair, comparable score of lean mass.
How it's calculated
FFMI = fat-free mass (kg) ÷ height (m)². A normalisation adjustment (+6.1 × (1.8 − height in m)) scales everyone to a common reference height so tall and short lifters can be compared.
The natural limit
Studies of pre-steroid-era bodybuilders and drug-tested athletes cluster around an FFMI ceiling of 25 for men. It's a useful reality check for setting goals, not an iron law.
Frequently asked questions
For men, an FFMI around 20 is average, 22–23 is very muscular, and ~25 approaches the natural limit. Women's scale runs several points lower.
Research on drug-free athletes suggests most men plateau near an FFMI of 25. It's a strong guideline but not absolute — genetics produce exceptions.
BMI uses total weight and can't distinguish muscle from fat. FFMI uses only fat-free mass, so it specifically reflects muscularity rather than overall size.