Not medical advice.These estimates are for general informational use only and aren't a substitute for professional medical advice, diagnosis, or treatment. Consult a qualified healthcare provider before changing your diet, exercise, or medication.

What FFMI measures

The Fat-Free Mass Index (FFMI) does for lean mass what BMI does for total weight: it divides your fat-free mass — everything that isn't fat, including muscle, bone, water and organs — by your height in metres squared. It was introduced by VanItallie and colleagues in 1990 as a way to compare lean mass between people of different heights, because raw kilograms of lean mass naturally favour taller people. From your weight, height and body-fat percentage we compute four things: lean mass, fat mass,raw FFMI, and a height-normalized FFMI.

How the numbers are calculated

Lean mass is your weight multiplied by (1 − your body-fat fraction); fat mass is the remainder. Raw FFMI is lean mass ÷ height². Because raw FFMI still drifts upward with height, we report a height-normalized FFMI that adjusts every result to a common 1.80 m reference, using the correction from Kouri et al. (1995):

normalized FFMI = raw FFMI + 6.3 × (1.80 − height in metres)

You may see a constant of 6.1 on other calculators; that is a later re-fit popularised online. We use 6.3 because it is the value published in the original paper. Near average heights the two barely differ — they are identical at exactly 1.80 m. All maths runs in metric internally; the interface converts your units at the edge.

How to read your result — and the limits of FFMI

Muscularity is a spectrum, not a pass/fail line. Our interpretation bands are descriptive reference points drawn from population data — Schutz et al. (2002) for general adults, Kouri et al. (1995) for trained men, and Harty et al. (2018) for trained women — and they are sex-specific because typical lean-mass levels differ between men and women. FFMI is only as accurate as the body-fat figure you feed it: a poor body-fat estimate produces a poor FFMI. It also cannot distinguish muscle from other lean tissue or account for hydration, frame size or genetics. The often-quoted “natural limit” near 25 for men comes from a single 1995 study of 157 athletes; it describes where most drug-free athletes in that sample topped out, not a ceiling you should chase or a verdict on anyone who exceeds it.

Your data stays on your device

Every calculation happens in your browser. Your weight, height and body-fat figures are never sent to a server, never stored remotely and never shared. If you save a result, it lives in this browser's local storage on this device only.

Frequently asked questions

What's the difference between raw and normalized FFMI?

Raw FFMI is simply your lean mass divided by your height squared. Because that figure rises with height even when muscularity is similar, normalized FFMI rescales the result to a standard 1.80 m reference so people of different heights can be compared fairly. At 1.80 m exactly the two values are identical; the further you are from 1.80 m, the more they diverge.

Is there really a “natural FFMI limit”?

The figure of about 25 for men comes from one 1995 study of 157 athletes, in which drug-free competitors clustered below ~25 while many steroid users exceeded it. For women, large athlete datasets put the practical ceiling around 21–22, with rare outliers higher. These are population observations, not hard biological walls and certainly not targets. Genetics, measurement error in your body-fat number, and your training history all move the figure, and exceeding a band does not prove anything about any individual.

Why are the bands different for men and women?

Women carry less skeletal muscle relative to height on average, largely due to hormonal differences, so the same FFMI value means something different by sex. We use sex-specific reference data rather than applying a single male-derived scale to everyone.

How accurate is my FFMI?

FFMI is only as good as the body-fat percentage you enter, and body-fat estimates carry real uncertainty. Treat your result as a ballpark for tracking change over time rather than a precise verdict. It also can't tell muscle apart from other lean tissue or account for hydration, frame or genetics.

Should I try to raise my FFMI?

FFMI is a description of where you are, not a goal you owe anyone. Building lean mass is a reasonable aim for many people, but the right approach is gradual training and adequate nutrition, not chasing a number. If a result leaves you feeling pressured about your body, that's worth taking seriously — see the support resources above.

Is my data saved or sent anywhere?

No. Every calculation runs in your browser, and your last inputs are stored only on this device so the form is ready next time. Nothing is uploaded and no account is required.