FFMIFat-free mass index

Fat-free mass index takes your fat-free mass in kilograms and divides it by height in metres squared — the same shape as BMI, but counting only the lean tissue. A normalised version adjusts to a standard height so people of different statures can be compared.

It matters because it separates muscularity from size in a way BMI cannot: a lean, heavily muscled person and someone carrying substantial fat can share a BMI while their FFMI differs greatly. It gives a single number for how much muscle you carry for your frame.

In practice the number is only as good as the body-fat estimate feeding it, so it inherits all the measurement error of whatever method produced that. It is also widely — and loosely — cited as a natural-limit threshold; the underlying research is far more tentative than that use implies, and it should not be treated as a test of anything about an individual.