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Lean body mass, FFMI and height-normalized FFMI, plus where you land against the natural-limit research. Free, no signup.
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These bands are a rule of thumb for men, built around one study: Kouri et al. (1995) measured male athletes who used anabolic steroids and those who did not. In that sample the nonusers' normalized FFMI topped out around 25, while many steroid users went past it. It is one study of men, with estimated body fat, so treat 25 as a rough marker of the natural range, not a hard limit.
| Normalized FFMI | Category | What it means |
|---|---|---|
| Under 18 | Below average | Lots of room to add muscle. |
| 18-20 | Average | Typical for men who do not lift seriously. |
| 20-22 | Above average | Consistent training shows here. |
| 22-23 | Excellent | Years of solid training. |
| 23-25 | Superior | Elite natural territory: rare and hard-won. |
| 25-26 | Near the natural ceiling | Right at the edge of what Kouri et al. saw in natural lifters. Possible but rare, and body fat error matters a lot here. |
| 26+ | Beyond the studied natural range | Above anything natural lifters showed in the study. More common with performance-enhancing drugs, but an understated body fat input or measurement error can also cause it. |
Source: Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine. 1995;5(4):223-228. Category labels are common community shorthand around that finding, not published clinical cutoffs.
Every input except body fat is easy to measure. A few points of error moves your FFMI by most of a point, which is why the result is only as good as your body fat estimate. No number yet? The AI body fat estimator gives you a range from a photo in seconds. It sends you back here with the number filled in.
FFMI (fat-free mass index) is your lean body mass in kilograms divided by your height in meters squared. It is like BMI, but it ignores fat, so a muscular person is not labeled "overweight" just for carrying muscle. Example: a man at 180 lb, 5'10" and 15% body fat has about 153 lb (69.4 kg) of lean mass and an FFMI of 22.0.
Normalized FFMI adds 6.1 x (1.8 minus your height in meters) to your FFMI. Taller people carry more lean mass per unit of height squared, so this scales them down and shorter people up, relative to a 1.8 m (5'11") person. Use the normalized number when comparing yourself to the research ranges. In the example above, the normalized FFMI is 22.1.
The commonly quoted figure is a normalized FFMI of about 25 for men. It comes from Kouri et al. (1995, Clinical Journal of Sport Medicine), which compared male athletes who used anabolic steroids with those who did not: the nonusers in that sample topped out around 25, while many steroid users exceeded it. It is a rule of thumb from one study of men, not a hard law. Body fat in studies and in real life is usually estimated, and some people argue a few natural lifters can exceed 25.
No. It is a signal, not proof. A body fat number that is too low, a measurement error, or unusually high lean mass can all push the result up. The reverse also holds: being under 25 does not mean someone is natural.
The calculator gives women lean body mass, FFMI and normalized FFMI, since the math is the same. But the Kouri et al. study only included men, and we do not cite an equivalent natural-limit study for women, so we do not show a "natural limit" category for women. Use the number to track your own progress over time.
The body fat input drives everything. A 3-point error in body fat shifts lean mass by about 3% of your bodyweight, which moves FFMI by roughly 0.7 points for someone at 180 lb and 5'10". If you do not know your body fat percentage, the free baisics body fat estimator can give you a range from a photo, and you can run both ends of that range here.
Lean body mass = bodyweight x (1 minus body fat percentage). Example: 180 lb at 15% body fat is 180 x 0.85 = 153 lb of lean mass and 27 lb of fat mass. Lean mass includes muscle, bone, organs and water, not just muscle.
Yes, it is free with no signup. The math runs in your browser and nothing is stored. Entering your email is optional and only needed if you want a program built around your numbers.