Muscle Index

FFMI is BMI with the fat thrown out. Your lean mass divided by your height squared, so it measures only what you have built. The number is then adjusted to a 1.8 m frame, because the same body reads differently on a taller one.

The reference points come from Kouri, Pope, Katz and Oliva (1995) in the Clinical Journal of Sport Medicine, who measured 157 male athletes — 83 steroid users and 74 non-users. The non-users fell between 16.6 and 25.0 once normalised.

That 25.0 is where this tool stops counting, and it is worth being precise about what it is: the top of what 74 men recorded in 1995, not a limit anybody proved. The paper never called it one. In the same paper the authors estimated the Mr. America winners from before steroids existed — the 1940s and 50s — at a mean of 25.4, above the number now quoted as the ceiling.

So this will not tell you whether somebody is natural, and it is not built to. Where that line falls moves with frame, genetics and years under the bar. It tells you where you are and how much room is above you.

One honest weakness: the answer is only as good as the body-fat figure you give it. Home scales and calipers are out by several percent, and three points of body fat move FFMI by about one. Treat your result as a range, not a reading — and if you guessed from the list, treat it as a wide one.

The reference sample was male, so the bands do not transfer to women. Rather than invent thresholds, the tool does not offer them.

Everything runs in your browser. No account, and your numbers are never sent anywhere. The only thing recorded is that the tool was run — a counter, nothing attached to you.