FFMI Calculator
With body weight 80 kg, height 1.8 m, body fat 15 %, ffmi comes to 21.0 kg/m² — ffmi. It is reached in 6 steps, the last of which is 68 / 1.8 ^ 2, and each one is printed on the page with its numbers filled in. The formula is the one published by Kouri et al., not an approximation fitted to it.
Fat-free mass index from weight, height and body fat, with the height-normalised version and the threshold the original study drew.
Formula and sources checked · How we check
Body weight 80 kg, Height 1.8 m, Body fat 15
21.0 kg/m²
FFMI for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Fat mass
80 * (15 / 100)12 kg- Fat-free mass
80 - 1268 kg- Fat-free mass index
68 / 1.8 ^ 220.988 kg/m²- Normalised to 1.8 m
20.987654 + 6.1 * (1.8 - 1.8)20.988 kg/m²- BMI, for comparison
80 / 1.8 ^ 224.691 kg/m²- Above the 25 threshold by
20.987654 - 25-4.012 kg/m²
Worked example
80 kg at 15% fat is 68 kg of fat-free mass, which over a height of 1.8 m squared is an FFMI of 21. At exactly 1.8 m the normalised figure is the same 21. BMI for the same person is 24.7 — the two indices differ by the fat, which is the entire point of using the first one.
How to work it out yourself
- 1.Multiply weight by body fat percentage for fat mass, and subtract it for fat-free mass.
- 2.Divide fat-free mass in kilograms by height in metres squared.
- 3.Normalise by adding 6.1 × (1.8 − height in metres), which removes the advantage short people get from the squared term.
- 4.Compare against BMI on the same person: the gap between them is the fat.
The formula
- Fat mass
80 * (15 / 100) - Fat-free mass
80 - 12 - Fat-free mass index
68 / 1.8 ^ 2 - Normalised to 1.8 m
20.987654 + 6.1 * (1.8 - 1.8) - BMI, for comparison
80 / 1.8 ^ 2 - Above the 25 threshold by
20.987654 - 25
Source: Kouri et al. — Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med 1995, NIH — assessing your weight and health risk
Questions people actually ask
- What is a good FFMI?
- For men, 18–20 is average, 20–22 is visibly muscular, and above 23 takes years of serious training. For women the same bands sit roughly 3 points lower. Unlike BMI, the index rewards muscle rather than penalising it, which is why it separates a lean athlete from an inactive person of identical weight.
- What is the FFMI natural limit of 25?
- It comes from Kouri and colleagues in 1995, who measured 157 male athletes and found that normalised FFMI in those who had never used steroids essentially stopped at about 25, while users went well above it. It is an observed ceiling in one sample, not a law of physiology, and a handful of very large-framed men exceed it naturally.
- Why normalise for height?
- Because dividing by height squared over-rewards short people — the same fault BMI has. The adjustment adds 6.1 for every metre below 1.8, which is the slope Kouri fitted. At exactly 1.8 m the raw and normalised figures are identical, which is a useful way to check you have entered height in the right units.
- How accurate is FFMI?
- Exactly as accurate as the body-fat figure that feeds it, and no more. A caliper reading can be five percentage points out, which on an 80 kg frame is four kilograms of assumed muscle and a full point of FFMI. DEXA or hydrostatic weighing narrows that; a bathroom scale with hand grips does not.
Related
- Body Fat CalculatorBody fat percentage from tape measurements using the US Navy circumference method, plus lean mass.
- BMI CalculatorBody mass index from height and weight in either metric or US units, with the CDC category cut-offs.
- Lean Body Mass CalculatorLean mass by the Boer, James and Hume equations side by side, with the fat mass each implies — the three disagree by kilograms, so the formula is the answer.
- Ideal Weight CalculatorReference weight from height by four standard clinical formulas, and the BMI-healthy range they sit inside.
- Army Body Fat CalculatorThe Army’s waist-to-height assessment that replaced the tape test in July 2026, with the waist measurement the 0.55 standard allows at your height.
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