FFMI Calculator

Fat-Free Mass Index (FFMI) does for lean mass what BMI does for total weight: it scales it to your height so people of different sizes can be compared. Because it's built from fat-free mass rather than total weight, it isn't inflated by body fat — which is why it's often used as a rough index of muscularity.

We report both the raw FFMI and a height-normalized version, which adjusts the figure to a standard height of 1.8 m so that taller and shorter people can be compared on the same scale. In men, a normalized FFMI around 25 is commonly cited as an approximate natural (drug-free) ceiling; the large majority of people sit comfortably below it. Women's typical values are lower, and there's no single agreed threshold.

FFMI is a descriptive index, not a health-risk score. It's most useful to serious trainees tracking whether they're gaining lean mass — provided the body-fat figure it's based on is measured consistently.

FFMI is most useful for people who train and track their body fat consistently — it's a good way to see whether lean mass is trending up over months, independent of fat changes.

Its biggest weakness is that it inherits all the error of the body-fat estimate it's built on, so a rough body-fat guess produces a rough FFMI. It's also less meaningful as a single snapshot than as a trend, the ~25 ceiling context applies to men rather than women, and it isn't a validated health measure. Keep the body-fat method the same each time and read FFMI as a relative number.

Formula. First we compute fat-free mass: FFM = weight × (1 − body fat% ÷ 100). Then FFMI = FFM(kg) ÷ height(m)². The height-normalized version adds 6.1 × (1.8 − height in m), following the method popularised by Kouri and colleagues. Imperial input is converted first.

Source. The FFMI framing and the height normalization come from Kouri et al. (Clinical Journal of Sport Medicine, 1995), the study that reported a normalized FFMI of roughly 25 as an approximate natural ceiling in men.

Confidence. We label FFMI moderate confidence — deliberately. The maths is simple, but the result is only as good as the body-fat input, which is itself an estimate, so the errors compound. It's a tracking tool, not a precise measurement.