Body & frame
FFMI (Fat-Free Mass Index)
Fat-Free Mass Index — lean mass in kg divided by height in meters squared, plus a height correction. The community's 'how big, drug-free' yardstick.
What FFMI (Fat-Free Mass Index) means
FFMI takes your lean body mass in kilograms, divides it by height in meters squared, then adds a height correction: FFMI = LBM/h² + 6.1 × (1.8 − h). An untrained man lands around 18-19; years of serious lifting typically buys 22-24. The famous ceiling of about 25 for drug-free trainees traces to Kouri et al. (1995), which compared steroid users to non-users — a real study, but a small sample of bodybuilders, not a law of biology. Drug-free outliers above 25 exist; they are just rare and usually large-framed.
What it actually does to the first impression
Nobody perceives an FFMI — what registers in 1.2 seconds is shoulder width, neck thickness, and how clothes hang, all of which FFMI only loosely drives. The visual difference between 19 and 22 is dramatic; between 23 and 25 it mostly shows shirtless. That makes FFMI a decent planning number and a bad obsession: past roughly 22, leanness moves the first-glance read more than additional mass does. A 22-FFMI man at 12% body fat out-photographs a 25 at 20% in nearly any clothed context.
Reality check: the forums vs the data
The community treats 25 as a hard natty-or-not detector, which Kouri's own data can't support — 157 men, mostly bodybuilders, with overlap between the groups. The other distortion: chasing FFMI rewards scale weight, so people bulk into 20%+ body fat and look worse at first glance than they did 8 kg lighter. FFMI also can't see frame; a long-clavicle man at 21 can out-silhouette a narrow one at 23. Useful once a year as a sanity check, useless as a weekly metric.
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Related terms
Reference data on this site