Last updated: 2026-07-08

Fat-Free Mass Index (FFMI)

The Fat-Free Mass Index (FFMI) expresses how much lean tissue a person carries relative to height. Fat-free mass — muscle, bone, organs, and water, everything other than fat — is divided by height in metres squared, giving a figure in kg/m². It is the lean-tissue counterpart of BMI Calculator: where body mass index uses total weight, FFMI uses only the part of weight that is not fat, so it reflects muscular development independent of how much fat someone carries.

What the index represents

Two people can weigh the same yet differ greatly in build. Body mass index treats them identically, because it knows nothing about composition. FFMI removes fat from the picture first and then scales what remains to height, so a heavier reading reflects more lean tissue rather than simply more weight. As a rough orientation, untrained men sit around 18 to 20 and untrained women around 14 to 16, with trained athletes ranging higher.

The formula

The calculation has three steps. First the fat-free mass, then the index, then a height adjustment:

Here $w$ is weight in kilograms, $\mathrm{BF}$ is the body-fat fraction, and $h$ is height in metres. The first line strips out the fat to leave the fat-free mass; the second scales it to height the way body mass index does. The third line is the height normalisation proposed by Kouri and colleagues: it adds about six tenths of a unit for every ten centimetres a person is shorter than 1.8 m, and subtracts the same for those who are taller. Without it, tall people tend to score lower for the same build, because lean mass does not rise exactly with the square of height.

Worked example

Consider an adult with a weight of 75 kg165 lb, a height of 1.80 m5 ft 11 in, and a body fat of fifteen percent. Working in metric units:

The fat-free mass is about 63.8 kg141 lb, the index is close to 19.7, and the adjusted value matches it because the height is exactly 1.8 m, where the correction is zero.

Interpreting the number

FFMI is read as a measure of lean development. Values near 19 to 20 are common for men with little training, while figures in the low twenties usually indicate meaningful muscle. A value of about 25 on the adjusted scale is a commonly cited ceiling for drug-free men with substantial muscle, taken from the Kouri study of bodybuilders, but it is a statistical observation rather than a hard threshold. Because the index is built from a body-fat estimate, its accuracy is no better than that estimate: an error in the body-fat reading passes straight into the result.

Limits

The index depends entirely on the inputs it is given, and the body-fat figure is the weakest link, since calipers, scales, and other methods disagree with one another. FFMI also says nothing about where lean mass sits or how it is distributed. It works best alongside related measures such as Lean Body Mass Calculator and Body Fat % Calculator (Navy Method), and any decision about training or health should rest on a professional assessment rather than a single index.