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FFMI calculator

Fat-Free Mass Index — an index of estimated fat-free mass relative to height, not a direct measurement of muscle. Enter your numbers below; the calculation runs in your browser.

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FFMI
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Normalized FFMI
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What FFMI actually is

FFMI is a height-adjusted index of estimated fat-free mass: fat-free mass divided by height squared. Fat-free mass is broader than skeletal muscle; it includes other non-fat tissues as well. The calculation uses a body-fat estimate, so uncertainty in that input carries into FFMI. The original Kouri study defines FFMI using fat-free mass.

Kouri and colleagues' 1995 study compared FFMI values in a sample of male athletes who reported steroid use and nonuse. The study is one source of the often-cited value near 25, but it does not make FFMI a direct muscle measurement or establish a universal natural limit. See the study's limits and context.

The formula

Two steps: take the fat out of your weight, then divide by height squared. Same shape as BMI — it just runs on the lean part only.

Fat-free mass
weight (kg) × (1 − body fat % ÷ 100)
FFMI
fat-free mass ÷ height (m) ²
Normalized FFMI
FFMI + 6.1 × (1.8 − height in m)
Worked example. 80 kg at 15% body fat, 1.75 m tall. Fat-free mass is 80 × 0.85 = 68 kg. FFMI is 68 ÷ (1.75 × 1.75) = 22.2. Then normalized: 22.2 + 6.1 × (1.8 − 1.75) = 22.5.
Why normalize at all? Lean mass scales faster than height squared, so dividing by height² under-corrects and quietly flatters taller lifters. The normalization adds a small amount back for anyone under 1.8 m, which is why it’s the normalized figure the bands below refer to — and the one this calculator grades you on.

What the number means

<18FFMI below 18 — a numerical range, not a label for training status or appearance.
18–20FFMI 18–20 — a numerical range; the result depends on body-fat and height inputs.
20–22FFMI 20–22 — a numerical range, not a direct measure of skeletal muscle.
22–23FFMI 22–23 — a numerical range; it cannot establish training history.
23–25FFMI 23–25 — a numerical range, not a population rank or natural-status classification.
25+FFMI 25 and above — a numerical result, not proof of steroid use or a biological ceiling.
These are simple display bands, not validated population categories. The 1995 study's sample does not establish universal ranges. Body-fat estimation, the normalization formula and other inputs affect the result. Treat FFMI as an index of estimated fat-free mass, not a muscle measurement or verdict about an individual.

Why it needs your body fat %

FFMI uses estimated fat-free mass, calculated here as weight × (1 − estimated body-fat percentage). Fat-free mass includes more than muscle. An uncertain body-fat input can move the result; try a range of plausible inputs and record the estimation method instead of treating the output as exact.

CutRank's first-angle scan gives a visual physique grade, not a body-fat estimate. Body-fat estimates are in paid reports; those are still image-based estimates, not measurements. See current report options and the measurement accuracy guide.

Questions

What is FFMI?
FFMI stands for Fat-Free Mass Index. It is estimated fat-free mass divided by height squared. Fat-free mass includes more than skeletal muscle, so FFMI is not a direct measure of muscle mass.
What is a good FFMI?
There is no single value that is "good" for everyone. FFMI is an index based on estimated fat-free mass and height; the estimate depends on its body-fat input and height-adjustment convention. It cannot determine health, training history or natural status.
Is 25 FFMI actually the natural limit?
No. The often-cited value of 25 comes from a small 1995 study of male athletes and is not a universal natural limit. The study reported a maximum normalized FFMI of 25.0 among 74 non-steroid users in its sample; it cannot establish a ceiling for all people or methods.
Why does FFMI need body fat percentage?
This calculator estimates fat-free mass by subtracting estimated fat mass from body weight. Fat-free mass includes more than skeletal muscle. An uncertain body-fat estimate makes FFMI uncertain too.

Want the full picture?

FFMI estimates fat-free mass relative to height. CutRank’s free first-angle grade gives a separate visual score for visible muscle groups.

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