FFMI (fat-free mass index) describes how much fat-free mass you carry relative to your height. It works like BMI but leaves out fat, so it separates muscle from fat.
Formula
FFMI = fat-free mass (kg) ÷ height (m)²
Taller people get slightly higher values for the same build. Kouri et al. 1995 therefore introduced a normalized FFMI that adjusts to a height of 1.80 m and makes values comparable.
Example: At 1.72 m, 75 kg and 12 percent body fat, your fat-free mass is 66.0 kg. Your FFMI is 66.0 ÷ 1.72² = 22.3, normalized about 22.8. This is a worked example.
What the limit of 25 means
Kouri et al. studied 157 male athletes, 83 of whom used anabolic steroids. Among non-users, normalized FFMI went up to about 25. That is a guideline from one sample, not a hard limit: the group was small, body fat was estimated from skinfolds, and some individuals are above it.
In practice, FFMI helps you decide whether a bulk is worth it. A low value means plenty of room to grow. If you are close to 25, every bulk becomes slow going. More useful than comparing yourself with others is your own trend: if your FFMI rises while body fat stays the same, you are adding fat-free mass. Keep in mind that it is only as accurate as your body fat estimate, and that the study only looked at men.