Fat-Free Mass Index

Fat-Free Mass Index (FFMI) scales a person's lean body mass by height squared, so it tells you how much non-fat tissue they carry relative to their body size. The height-squared term matters because larger people naturally weigh more even if they are not more muscular; dividing by h² makes the number more comparable across different heights, much like BMI does for total mass. Because it uses lean body mass rather than total body mass, FFMI is more focused on muscularity and structural tissue than on body fat.

Use FFMI when you want to compare muscularity between people of different heights, such as in sports science, physique assessment, or training research. It is also useful for tracking changes in muscle gain over time when body fat is changing at the same time.

• Using total body mass instead of lean body mass
• Forgetting to square height, or using height in centimetres without converting to metres
• Interpreting FFMI as a direct measure of body fat or overall health