Fat-Free Mass Index

FFMI: a smarter BMI for lean mass

BMI treats a powerlifter and a couch potato of the same weight as identical. The Fat-Free Mass Index fixes that by dividing lean body mass (not total mass) by height squared. The formula is FFMI = (weight · (1 − %fat/100)) / height², weight in kg and height in m. Take someone at 80 kg, 15% fat, 1.78 m: that's 68 kg of lean mass and an FFMI near 21.5. For men, roughly 18 reads as sedentary, 19–22 as trained, 22–24 as advanced natural, and about 25 marks the practical ceiling for drug-free lifters under the Casey Butt model. Once you see 25.5 or higher, anabolic use becomes the likely explanation. Women sit around 3 points lower, with 14–17 counting as natural and 18–20 as advanced. It's the number people reach for in the "natty or not" debate and when tracking a body recomposition.

Applications

Sports nutritionists and physique coaches lean on it, as does anti-doping reasoning in bodybuilding. Geriatrics uses it to screen for sarcopenia, where a low FFMI predicts mortality better than a low BMI does. You'll also find it in eating-disorder rehabilitation tracking and in anthropometric research.

FAQ

How do I know my body-fat percentage? A DEXA scan is the gold standard. Bioimpedance scales and skinfold calipers are easier to get hold of but less accurate, and an error of ±3% feeds straight through into your FFMI.

Is there a normalized FFMI? There is. Kouri's adjustment tacks on 6.1·(1.8 − h) so you can compare people of different heights, since taller lifters tend to carry a bit less relative lean mass.

Can FFMI exceed 25 naturally? A few documented cases do exist, mostly genetic outliers and elite strength athletes. But when you survey drug-tested competitors, FFMI rarely climbs past 25, which is where the "natty ceiling" rule of thumb comes from.