FFMI (Fat-Free Mass Index) = lean body mass (kg) ÷ height (m)². It measures the amount of lean mass relative to height, analogous to BMI but for muscle. A natural male bodybuilder rarely exceeds FFMI 25; values above 25 are extremely rare naturally and are associated with anabolic steroid use in research. Average untrained male FFMI is approximately 19-20; trained naturals typically reach 21-24. The metric was popularised in a 1995 study comparing modern bodybuilders to pre-steroid era athletes. Female FFMI values are approximately 4 points lower.
Key points
- Lean mass (kg) / height (m)²
- Male natural ceiling ~25 FFMI
- Average trained natural male 21-24
- Values above 25 very rare without pharmaceutical assistance