FFMI Calculator — Fat-Free Mass Index and the Kouri 25 Anchor

What FFMI Is For

BMI cannot tell a muscular 90 kg from an unfit 90 kg, because it only knows total mass. FFMI fixes that by throwing away the fat:

FFM = weight × (1 − bodyFat / 100) FFMI = FFM / height²

That is a direct measure of how much lean tissue you carry for your frame — the number BMI was always being misused as a proxy for.

Why Normalise It

Raw FFMI still rewards being short. The same lean mass divided by a smaller height² produces a bigger index, so a 1.65 m and a 1.85 m lifter with identical body composition do not get comparable numbers. The normalised form rescales both to a 1.8 m reference:

Normalised FFMI = FFMI + 6.1 × (1.8 − height_m)

Every published reference figure — including the 25 everyone quotes — is in the normalised form. Compare that one, not the raw.

What Kouri 1995 Actually Found

The FFMI 25 threshold traces to one paper: Kouri and colleagues, *Clinical Journal of Sport Medicine*, 1995. They measured 157 male athletes, 83 of whom used anabolic steroids and 74 of whom did not. The non-users reached a normalised FFMI of about 25 and no higher; a large share of the users exceeded it.

Three things get lost when that becomes "25 is the natural limit":

1. The authors did not claim a ceiling. They described the work as preliminary and proposed 25 as a *screening* threshold — a flag worth a second look, not a law of physiology. 2. The sample was 74 non-users. The highest value in a sample of 74 is not the highest value attainable by humans. It is the highest value in that sample. 3. Later data complicates it. A collegiate-athlete sample has been reported with a 97.5th percentile of 28.3 for men, which is well past the line.

Above 25 is unusual. It is not evidence of anything on its own.

The Body Fat Number Decides Everything

FFMI inherits every error in the body fat figure you feed it. A tape-measure estimate carries ±3–4% against DEXA, and a 4-point error moves normalised FFMI by roughly 1.0 — enough to move you a whole band. If you are going to act on the result, measure body fat properly first; if you only have a tape, use the body fat calculator and treat the FFMI as a range rather than a point.

Not the Same as a Potential Estimate

FFMI measures what you carry now. The natural muscle potential calculator does something different: it predicts a *ceiling* from frame measurements using the Casey Butt regression. One is a measurement, the other a projection, and a tool that claims to be both is doing neither properly.

Women and This Scale

The Kouri sample was male only, and no equivalent threshold study exists for women. The female bands in this calculator are descriptive — useful for orientation, but no line on them carries the evidence the male 25 does. Treating a women’s FFMI of 20 as "the female natural limit" is an extrapolation nobody has published.