Fat-Free Mass Index

What FFMI is measuring

BMI divides your whole bodyweight by your height squared, so it cannot tell muscle from fat. FFMI runs the same division on your fat-free mass alone, which is your weight minus the share of it that is body fat. Strip the fat out of the top of the fraction and what is left describes how much lean tissue you carry for your height, which is a genuinely different question. The adjusted figure goes one step further and normalizes to a 1.8 metre frame so a shorter and a taller person can be read on the same scale.

Where the ranges came from

Most of the numbers you see quoted trace back to one paper: Kouri and colleagues in 1995, who measured fat-free mass in male athletes who had used anabolic steroids and those who had not, and added a group of Mr. America winners from 1939 to 1959, before those drugs were in circulation. The drug-free groups almost never landed above an FFMI of about 25, and the figure of roughly 25 to 26 has been repeated as a natural ceiling ever since. It is a useful reference point, and it is also one modest study of men.

A worked example

Say you are 5 foot 10, which is 1.78 metres, weigh 180 pounds, and your body fat estimate is 15 percent. That leaves about 153 pounds of lean mass, or 69.4 kilograms. Square the height to get 3.16, divide, and your FFMI is about 22.0. The height correction adds a tenth or so because you are just under 1.8 metres, so the adjusted figure comes out near 22.1 and the tool labels that band Excellent. The important part is the lean mass line, since that is the number a few months of training can actually move.

Where the number gets over-read

The ceiling is an observation about a population, not a rule your body agreed to, and drug-free lifters do turn up above it. The height correction is a straight-line adjustment that gets shaky at the extremes of very short and very tall. Most of all, FFMI inherits every bit of error in your body fat estimate: if that estimate is off by three points, the FFMI in the example above moves about three-quarters of a point on its own. Treat it as a rough position, not a grade, and never as a verdict on how anyone should look.

What to do with your number

Improve the input before you argue with the output. A tighter body fat estimate does more for the accuracy of this reading than anything else, and your lean body mass in pounds is the friendlier way to watch progress month to month. Run it again every two or three months during a gaining phase and see whether the weight you added came with the lean mass. That trend is the useful signal. The single reading is just where you happen to be standing today.