FFMI Calculator: Fat-Free Mass Index and the Natural Limit

Short answer

FFMI (fat-free mass index) is your lean mass divided by your height squared, a bit like BMI but without the fat. Example with made-up numbers: 78 kg, 1.75 m and 15% body fat give 66.3 kg of lean mass, an FFMI of 21.65 and a height-adjusted FFMI of 21.95.

The formulas

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

FFMI and the height correction come from a study of strength athletes by Kouri et al. (1995). The correction puts everyone on the scale of a 1.8 m man: it adds a little for shorter people and takes a little off for taller ones. The paper's abstract prints the factor as 6.3, while most calculators, this one included, use 6.1. Between 1.5 and 2.1 m the two versions differ by less than 0.1 points.

In the example: 78 × 0.85 = 66.3 kg of lean mass. 66.3 ÷ 1.75² = 21.65. Adjusted: 21.65 + 6.1 × 0.05 = 21.95.

Lean mass for a given FFMI

How much lean mass it takes to reach a given (unadjusted) FFMI at different heights:

Where the "natural limit of 25" comes from

Kouri and colleagues measured 157 male athletes: 83 who used anabolic steroids and 74 who didn't. In the non-users, adjusted FFMI went up to a limit of about 25.0. Twenty Mr. America winners from 1939 to 1959, before steroids were around, had an estimated average of 25.4. Many of the users were above 25, and some above 30. The authors called the results preliminary and suggested FFMI as a first screening tool, not as proof.

A few things to keep in mind before you use 25 as a verdict on yourself or anyone else:

• The pre-steroid champions averaged slightly over 25, so some drug-free people sit above it.
• The sample was small and only included men. There is no equivalent cutoff for women from this study.
• FFMI depends on your body fat estimate. At 78 kg and 1.75 m, every percentage point of body fat changes FFMI by about 0.25. The Navy tape method, for example, has a standard error of about 2.7 points in men and 4 in women, which moves FFMI by roughly 0.7 to 1 point either way.

So an FFMI above 25 doesn't prove drug use, and one below 25 doesn't prove anything either.

How to use it

FFMI works best as a personal tracking number. Measure body fat with the same method and under the same conditions every time, and look at whether your lean mass and FFMI go up over months of training. If your FFMI rises while your body fat stays the same, the extra weight is mostly lean mass.