FFMI Calculator (Fat-Free Mass Index)

About the FFMI Calculator (Fat-Free Mass Index)

BMI cannot tell muscle from fat. The fat-free mass index (FFMI) can: it is your fat-free mass — everything that is not fat, mostly muscle, bone, organs and water — divided by your height squared. Its partner, the fat mass index (FMI), does the same for fat, and the two add up to your BMI.

Enter your weight, height and body-fat percentage to get your fat-free and fat mass, FFMI, FMI and the height-normalised FFMI used in sports research. If you add your age, the calculator shows where you sit among adults of your sex and age group in a national reference survey, next to the threshold doctors use for low muscle mass — and it explains what the often-quoted “natural limit” of 25 really is.

The formulas

• Fat mass (kg) = weight × body fat % ÷ 100; fat-free mass = weight − fat mass
• FFMI = fat-free mass (kg) ÷ height (m)²
• FMI = fat mass (kg) ÷ height (m)², and FFMI + FMI = BMI
• Height-normalised FFMI = FFMI + 6.3 × (1.80 − height in m)

The normalisation comes from Kouri et al., who adjusted every value to that of a 1.80 m man because taller people tend to have a slightly higher FFMI. The 6.3 factor is the one given in the paper’s abstract. You will also see 6.1 quoted; between 1.30 and 2.30 m tall the two give results no more than 0.1 apart.

What the “natural limit” of 25 is — and isn’t

Kouri and colleagues measured 157 male athletes: 83 who used anabolic steroids and 74 who did not. The height-normalised FFMI of the non-users extended up to a well-defined limit of 25.0, while many users exceeded it and some passed 30. Twenty Mr America winners from the pre-steroid era (1939–1959) had an estimated mean of 25.4.

That is a population observation from one small study of men, not a rule. Body-fat estimates can easily be a few percentage points off, which moves FFMI by around 0.5–1, and some people have unusual builds. A value above 25 proves nothing, and a value below it rules nothing out. No comparable limit has been published for women.

Reference values

• Percentiles by age group: the comparison uses Table 2 of Kim et al., percentiles of FFMI measured with multifrequency bioimpedance in a nationally representative survey of 10,140 people in Korea (KNHANES 2022–23). For men aged 30–39 the median is 19.0 and the 5th–95th percentiles are 16.1–22.5; for women of the same age, 15.2 and 13.3–18.6.
• Swiss adults: Schutz, Kyle and Pichard measured 5,635 healthy adults in Switzerland with bioimpedance; the median FFMI at 18–34 years was 18.9 in men and 15.4 in women.
• Low muscle mass: the GLIM criteria for diagnosing malnutrition list an FFMI (by bioimpedance) below 17 in men and 15 in women as an example threshold for low muscle mass.

Reference ranges differ by population, age and measuring method, so treat the comparison as a guide.

How accurate is it?

FFMI is only as good as the body-fat percentage you enter. DXA scans and skinfolds taken by a trained person are the most reliable; tape-measure methods and body-composition scales can be several points out. For an 80 kg person, every 1 point of body fat is 0.8 kg of fat-free mass — about 0.25 of FFMI at 1.80 m. The calculator shows your FFMI 3 points either side of your body-fat figure so you can see the range.

Sources

• Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 5:223-8
• Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. Int J Obes 26:953-60
• Kim H, Hong YH, Jang YC, et al. National reference values of FFMI and FMI using body composition chart in Korean adults. Nutrients 18:1170 (CC BY 4.0) — Table 2
• Jensen GL, Cederholm T, et al. GLIM consensus approach to diagnosis of malnutrition: a 5-year update. JPEN 49:414-27 — Table 2

Limitations

• The result depends entirely on the body-fat percentage entered; a few points of error moves FFMI noticeably.
• The age-group percentiles come from Korean adults measured by bioimpedance; other populations and DXA-based studies give somewhat different values.
• Not for children, teenagers or pregnancy. Fluid retention (oedema) inflates fat-free mass.
• Weights of 25–300 kg, heights of 120–230 cm and body fat of 2–70% are accepted.

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