FFMIFat-free mass index
A muscularity index: fat-free mass adjusted for height, the lean-mass answer to BMI.
Fat free mass index is fat free mass in kilograms divided by height in metres squared. It is BMI with the fat removed, which makes it a measure of how much lean tissue you carry for your height rather than how much you weigh.
It matters because it separates the two things BMI conflates. A muscular person and a person carrying substantial fat can share a BMI; their FFMI values differ substantially, which is why the index is used both in sports science and in clinical assessment of muscle wasting.
Its most quoted application comes from a 1995 study of 157 male athletes. Normalised FFMI in those who had never used anabolic steroids extended up to a well defined limit of 25.0, while users commonly exceeded it, which is where the natural limit figure in gym discussion originates.
That figure deserves care. It is one cohort from one study three decades ago, it is a statistical observation rather than a physiological law, and applying it to an individual as proof of anything is a misuse of it. Very tall or very short people are also poorly served by height squared scaling.
It depends entirely on the body fat estimate underneath it. Feed it a bioimpedance reading taken after a salty meal and the output inherits that error, which is why consistency of method matters more than the absolute number.
For context, roughly 18 to 20 reflects an average untrained man, 20 to 23 serious training, and 23 to 25 approaches the upper end of what the original cohort showed.
In practice, use our FFMI calculator to track your own trend over years, and treat the 25 line as a landmark rather than a verdict.