How FFMI is calculated
Lean mass = weight × (1 − body fat %). FFMI = lean mass (kg) ÷ height (m)². The normalized version adds 6.1 × (1.8 − height in m), which corrects for taller people naturally scoring a little higher.
The "natural limit"
A well-known 1995 study of male athletes found that drug-free lifters rarely exceeded a normalized FFMI of about 25. It's a useful reference point, not a hard rule: genetics vary, and body fat estimates have error that shifts FFMI too.
Why FFMI beats BMI for lifters
BMI counts all weight the same. FFMI only counts lean mass, so it shows whether added weight is muscle, which makes it a better progress marker if you're training to build muscle.