Glossary
What is FFMI?
Short answer
FFMI (Fat-Free Mass Index) is your fat-free mass in kilograms divided by your height in metres squared. It works like BMI, but counts only the part of your weight that isn't fat, so it tracks muscularity relative to your frame instead of total size.
Also called: Fat-free mass index · Lean mass index
Felipe PassosLast reviewed
The formula
FFMI starts with fat-free mass: everything in your body that isn’t fat, including muscle, bone, organs and water.
Fat-free mass (kg) = weight (kg) × (1 − body fat % ÷ 100)
FFMI = fat-free mass (kg) ÷ height (m)²Most calculators then apply a height correction, so a tall and a short person carrying proportionally similar muscle land on similar numbers:
Normalized FFMI = FFMI + 6.1 × (1.8 − height in m)Example: 80 kg at 16% body fat and 1.78 m gives 67.2 kg of fat-free mass, an FFMI of 21.2 and a normalized FFMI of 21.3. The FFMI calculator does this for your own numbers.
Where “FFMI 25” comes from
The number people quote comes from a 1995 study by Kouri and colleagues. They calculated FFMI for 157 male athletes: 83 who used anabolic-androgenic steroids and 74 who didn’t. The normalized FFMI of the non-users extended up to a limit of about 25.0. Many steroid users exceeded that, some past 30.
The authors called their findings preliminary. The sample was male-only and drug use was self-reported. A value above 25 is not proof of anything, and a value below it is not a ceiling you are guaranteed to reach.
What FFMI can and can’t tell you
FFMI answers “how much lean mass do I carry for my height?” better than weight or BMI. BMI can’t tell muscle from fat.
It can’t go further than that:
- It isn’t a muscle measurement. Fat-free mass also includes bone, organs and water.
- It inherits the error of your body-fat estimate. A 3-point error in body fat moves FFMI by close to a full point. Change how you measure body fat and your FFMI jumps, even if your body didn’t.
That’s why FFMI is most useful as a long-term trend measured the same way each time, rather than a single score. The full FFMI guide covers worked examples, FFMI for women, and how Corpus tracks it at every check-in.
Go deeper
Common questions
What is a good FFMI?
There's no universal 'good' value. In the study most charts are based on, the normalized FFMI of male athletes who didn't use steroids extended up to about 25. Most people who train regularly sit well below that, and a rising personal trend matters more than where you land on a chart.
What is the difference between FFMI and normalized FFMI?
Normalized FFMI adds a small height correction so people of different heights compare more fairly. The common version is FFMI + 6.1 × (1.8 − height in metres); the original 1995 paper used 6.3, which changes results by only a few hundredths of a point at typical heights.
Does FFMI work for women?
The formula works for anyone, but the reference values that give FFMI numbers meaning come mostly from male samples. For women, treat published cutoffs with extra caution and focus on your own trend.
Sources
- Fat-free mass index in users and nonusers of anabolic-androgenic steroids · Clinical Journal of Sport Medicine / PubMed
- Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y · International Journal of Obesity / PubMed
- Current body composition measurement techniques · Current Opinion in Endocrinology, Diabetes and Obesity / PubMed
Educational content only; not medical advice or individualized professional guidance.
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