Fat-free mass index describes how much lean tissue you carry relative to your height. BMI cannot distinguish muscle from fat, so a lean, muscular person and a sedentary one of the same weight receive identical scores. FFMI separates them.
The calculation takes your weight and body fat percentage to find fat-free mass, then divides by height squared. A normalised version adjusts to a standard height so that tall and short people can be compared fairly.
Rough guidance for men: below 18 is below average, 18 to 20 average, 20 to 22 above average, 22 to 23 well muscled, and above 23 exceptional.
For women the scale sits lower, roughly 3 to 4 points below at each level, because of natural differences in body composition.
The figure most people come here for is around 25. Analysis of drug-free bodybuilders from the pre-steroid era found almost none exceeded it, and it has been used ever since as an approximate ceiling for natural muscularity.
The 25 figure comes from a 1995 study of 157 men, including 83 who used steroids and 74 who did not. The natural group clustered below 25; the steroid users routinely exceeded it.
Two cautions before you treat it as a verdict.
First, it is a population observation, not a physical law. Some people with exceptional genetics and long training histories exceed it naturally, and the original authors were careful to say so.
Second, and more importantly, the result depends entirely on your body fat estimate. Underestimate body fat and you overestimate lean mass, inflating FFMI. Callipers and bioimpedance scales can be several percentage points out, and that error carries straight through.
So an FFMI of 24 from a bathroom scale is not evidence of anything. An FFMI of 24 from a DEXA scan is worth taking seriously.
Tracking yourself over time, using the same measurement method each time. Absolute accuracy matters less than consistency when you want to know whether you are gaining muscle or losing it.
It is also a useful reality check on claims. A physique presented as natural with a calculated FFMI well above 25 is worth questioning.
What it is not useful for is judging health. FFMI describes muscularity, and muscularity is not the same as fitness or wellbeing.
Related reading: How much muscle can you build naturally · Does lifting increase testosterone · Anabolic steroid side effects · Body fat calculator
Educational only, not medical advice.