Fat-Free Mass Index (FFMI) Calculator

Understanding the FFMI calculator — concept and principle

The fat-free mass index (FFMI) measures how much lean tissue you carry for your height. It works like BMI but leaves fat out, so it separates a muscular person from one who simply weighs more. Strength athletes and bodybuilders use it to track muscle gain, and researchers have used it to study the limits of drug-free muscular development. This calculator needs your weight, height and a body fat estimate.

Lean mass per square metre of height: 21.12 after adjusting to 1.8 m, well under the ~25 guideline.

How it is calculated

Lean mass (kg) = weight × (1 − body fat % ÷ 100).
FFMI = lean mass ÷ height (m)².
Normalised FFMI = FFMI + 6.1 × (1.8 − height in m).

Normalisation adjusts the value to what it would be at a height of 1.8 m, so people of different heights can be compared.

The principle

Dividing by height² removes most of the effect of body size, just as in BMI. In practice, though, taller lifters still tend to score slightly lower FFMI at the same level of development, so Kouri and colleagues added a correction of 6.1 points per metre of height difference from 1.8 m. In their 1995 study of 157 male athletes, men who had never used anabolic steroids had normalised FFMIs up to about 25, while many steroid users exceeded it. That observation is the source of the often-quoted "natural limit" of 25. FFMI is only as accurate as the body fat figure you enter.

Worked example

Inputs: weight 75 kg, height 175 cm, body fat 15%.
Lean mass = 75 × (1 − 0.15) = 63.75 kg.
Height² = 1.75² = 3.0625, so FFMI = 63.75 ÷ 3.0625 = 20.82.
Normalised FFMI = 20.82 + 6.1 × (1.8 − 1.75) = 20.82 + 0.31 = 21.12.
21.12 is below 25, so the note reads within the natural range.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Normalised FFMI
43.8	341.37
87.5	88.91
131	39.98
175	21.12
219	10.96
262	4.22
350	-5.17

How to read the result

Higher values mean more lean mass for your height. A score around 25 or more represents a level of muscularity that few drug-free men reach, but it is not proof of steroid use; some exceptional athletes exceed it, and errors in body fat estimates easily move FFMI by a point. Use FFMI mainly to follow your own progress: at constant height, a rise of one point equals about 3 kg of lean mass for a 1.75 m person. Women's values are generally lower than men's, and the cut-off of 25 was derived from men only.

Common mistakes

Entering an inaccurate body fat percentage; a 5-point error in body fat changes FFMI by about 1.2 at 75 kg and 1.75 m.
Comparing your raw FFMI with the normalised cut-off of 25.
Using the 25 rule to accuse someone of drug use.

Keep in mind: FFMI is an estimate based on another estimate (body fat). It is not a diagnosis and does not measure health. Talk to a doctor or qualified coach before making major changes to training or diet.

Frequently asked questions

What is a good FFMI for men?

There is no official grading scale. The normalised value of about 25 from Kouri's study is the main reference point.

Can women use FFMI?

Yes, the formula is the same, but expect lower values; the natural-limit figure of 25 does not apply to women.

Does FFMI include bone and water?

Yes. Lean mass here is everything except fat, so it includes bone, organs and body water as well as muscle.

Sources: 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 5 (1995) 223–228

Checked 8 Nov 2026. Kouri, Pope et al., Clin J Sport Med 1995.

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