Fat-Free Mass Index (FFMI) Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
FFMI is calculated with fat-free mass (kg) ÷ height (m)². Its normalized version adds a correction of 6.1 × (1.80 − height) to compare different body sizes. A 90 kg man at 15% body fat and 1.80 m has an FFMI of 23.6, within the usual range of a well-trained natural lifter.
Explanation
FFMI (Fat-Free Mass Index) relates muscle and bone mass (fat-free mass, i.e. total weight minus fat mass) to height squared — the same principle as BMI, but applied only to fat-free mass rather than total weight, which makes it far more relevant for assessing a level of muscular development. Since two people with the same raw FFMI but very different heights don't have exactly the same relative muscle mass (a taller person naturally carries more fat-free mass in absolute terms), a normalized version brings everyone back to a reference height of 1.80 m via the correction + 6.1 × (1.80 − height), allowing a fairer comparison between different body sizes. FFMI is best known for its use as a plausibility benchmark in natural bodybuilding: nearly all published data on non-doped athletes places normalized FFMI between 21 and 23, with values above 25 being rare and those exceeding 28 exceptional for a trained man without pharmacological assistance — which makes it an indicator used (with its limitations) to flag a likelihood of doping in some natural bodybuilding federations. **This reference threshold around 25 is established on an overwhelmingly male population**: the literature does not provide a solidly established equivalent threshold for women, whose muscle physiology differs noticeably. To get the body fat percentage needed as input, if you don't already know it, see our body fat calculator.
Example: 90 kg, 15% body fat, 1.80 m
Inputs
Weight: 90 kg. Body fat percentage: 15%. Height: 1.80 m.
Calculation
Fat-free mass = 90 × (1 − 0.15) = 76.5 kg. FFMI = 76.5 ÷ 1.80² = 76.5 ÷ 3.24 ≈ 23.61. At exactly 1.80 m, the normalization correction is zero: normalized FFMI = 23.61.
Result
This FFMI of 23.6 falls in the high but plausible range for a well-trained natural lifter.
Frequently asked questions
Does an FFMI above 25 automatically mean doping?
No, it is never proof on its own: a high FFMI can also be explained by exceptional muscle genetics, a particular bone structure, or simply an imprecise measurement of body fat percentage (an error of a few points in this estimate noticeably changes the result). An FFMI above 25 is rare but not impossible in a natural athlete, and serves more as a statistical plausibility benchmark than a definitive test.
Why normalize FFMI rather than keep the raw value?
Because for proportionally identical fat-free mass, a taller person has a slightly lower raw FFMI than a shorter person, a mechanical effect of squaring height that has nothing to do with actual muscular development. The normalized version corrects for this effect by bringing everyone back to a common reference height (1.80 m), for a fairer comparison between body sizes.
Is FFMI as reliable as the Wilks score for comparing athletes?
These are two different indicators that don't measure the same thing: the Wilks score compares a strength performance (a total load lifted) between different body sizes, while FFMI compares a level of muscular development independent of any performance. A high FFMI does not necessarily imply a high Wilks score, and vice versa.