Fat-Free Mass Index: muscularity adjusted for height
FFMI takes the same idea as BMI — normalizing a body measurement by height — but applies it to fat-free mass instead of total weight. The result is a number that lets you compare relative muscularity between people of different heights more fairly than total lean mass alone would.
FFMI = lean body mass(kg) ÷ height(m)²
Normalized FFMI = FFMI + 6.1 × (1.8 − height(m))
The normalization term corrects a known quirk where taller people score slightly higher FFMI at the same relative muscularity as shorter people — without it, cross-height comparisons would be subtly unfair.
Where the reference ranges come from
FFMI gained attention in sports science research examining the upper limits of natural (drug-free) muscle growth. Several studies surveying competitive natural bodybuilders found normalized FFMI rarely exceeding the mid-20s, which is part of why the metric is sometimes informally used as a rough sanity check, though it's far from a definitive test and individual genetic variation is substantial.