FFMI Calculator — Fat-Free Mass Index

BMI has a well-known blind spot: because it works from total weight, it cannot separate muscle from fat, so a muscular person is routinely labelled “overweight” on a scale that only sees kilograms. FFMI was built to close that gap. It takes the same height-squared normalisation that makes BMI comparable across tall and short people, but feeds in only your fat-free mass — everything that is not fat, meaning muscle, bone, organs and the water they hold. The result is a single figure that describes how much lean tissue you carry relative to your height, independent of how much fat sits on top. That is why FFMI is the more useful index for anyone who lifts, and why it tends to move as you build or lose muscle rather than as the scale drifts.

The arithmetic is short. Take someone who weighs 80 kg at 15% body fat and stands 1.80 m tall. Their lean mass is 80 × (1 − 0.15) = 68 kg. Square the height: 1.80 × 1.80 = 3.24. Divide: 68 ÷ 3.24 = an FFMI of about 21.0 kg/m². That is the whole calculation, and it is exactly what the tool above does once you enter your numbers. If you do not know your body fat, the calculator first estimates your lean mass from height, weight and sex with the Boer formula and then runs the same divide-by-height-squared step, so you still get an FFMI — just built on an estimate rather than a measurement.

Across untrained adults, FFMI figures are commonly reported roughly in the mid-to-high teens for women and around the high teens to low twenties for men, drifting higher in people who carry more muscle. Those are descriptions of what is commonly observed, not targets to hit — FFMI is a way to track your own lean mass over time, and the honest use of it is to watch your own trend rather than to compare yourself to a threshold. Because it is derived from an estimate of lean mass, treat a single reading as a ballpark, not a precise verdict. The contrast with BMI is the whole point: BMI answers “how heavy am I for my height?” while FFMI answers “how much lean tissue do I carry for my height?” The two often disagree for the same person, and when they do, FFMI is the one that reflects training. To see the fat side of the same body, use the BMI Calculator alongside this page, and measure your body fat directly with the Body Fat % Calculator for the most accurate FFMI. The lean mass FFMI is built on comes straight from the Lean Body Mass Calculator.