FFMI (Fat-Free Mass Index)

FFMI: the "natty ceiling" index

The Fat-Free Mass Index measures your lean body mass against height squared, so it tells you about muscle without fat muddying the picture. Formula: FFMI = (weight · (1 − %fat/100)) / height², weight in kg and height in m. There's a height-adjusted version that tacks on 6.3·(1.8 − h) so you can compare people of different heights fairly. Take 80 kg, 15% fat, 1.75 m: lean mass works out to 68 kg and FFMI to roughly 22.2. For men, around 18 reads as sedentary, 19–22 as trained, 22–24 as advanced natural, and 25–26 as about the practical ceiling for drug-free lifters (that's from Casey Butt's data on pre-steroid-era champions). A score of 25.5 or higher is a strong hint of anabolic use. Women sit lower across the board: 14–17 is average, 17–19 trained, and about 20 the natural ceiling.

Applications

Tracking body recomposition. Settling the "natty or not" arguments in amateur bodybuilding. Setting benchmarks for physique coaches. Backing up anti-doping reasoning. And anthropometric research where lean mass tells you more than total weight does.

FAQ

How accurate must body-fat % be? Pretty accurate. Be off by ±3% on the fat reading and your FFMI moves about 0.7 points, which can be enough to bump you into another category. DEXA is the gold standard. Bioimpedance and skinfolds get the job done but are noisier.

Why height-adjust? For the same amount of training, taller people end up carrying a little less lean mass in relative terms. The +6.3·(1.8 − h) term pegs everyone to a 1.80 m reference so the comparison holds up.

Can someone naturally exceed 25? There are documented outliers, like genetic responders and elite strongmen. Still, in drug-tested groups an FFMI above 25 is rare, and that's why people treat the number as a soft ceiling and not a hard cutoff.