What FFMI measures
Fat-free mass index (FFMI) is like BMI, but it counts only your fat-free mass: muscle, bone, organs and water. Two people with the same BMI can have very different FFMIs, because one may carry more muscle and the other more fat. That makes FFMI a better way to describe how muscular someone is for their height.
How it's calculated
The formulas come from Kouri and colleagues (1995), who used FFMI to study athletes. Weight is in kilograms and height in meters:
FFMI = fat-free mass ÷ height²
Normalized FFMI = FFMI + 6.1 × (1.8 − height)
The normalized FFMI adjusts to a height of 1.8 m (about 5 ft 11 in). Taller people tend to have a slightly higher FFMI, so the correction lets people of different heights be compared. The authors described the 6.1 factor as an approximate correction. US units are converted with 1 lb = 0.45359237 kg and 1 in = 2.54 cm.
Worked example
A man 5 ft 10 in (1.778 m) who weighs 180 lb (81.65 kg) at 15% body fat:
FFMI = 69.40 ÷ 1.778² = 21.95, about 22.0
Normalized FFMI = 21.95 + 6.1 × (1.8 − 1.778) = 22.09, about 22.1
How to read your FFMI
There are no official health categories for FFMI. Two published reference points give useful context:
- In a sample of over 5,000 healthy Swiss adults, the median FFMI at ages 18 to 34 was 18.9 for men and 15.4 for women (Schutz and colleagues, 2002).
- In Kouri's study of 157 male athletes, the normalized FFMI of men who had not used steroids went up to a well-defined limit of about 25. The authors called their findings preliminary, and later writers have questioned treating 25 as a hard limit.
Use FFMI to track your own progress over time rather than to judge yourself against a single number.
Limitations
- It depends on your body fat number. Scales, calipers and tape methods can be off by several percentage points, which changes FFMI noticeably. Use the same method each time.
- The height correction and the 25 reference come from men. They may not suit women or teenagers.
- FFMI says nothing about strength, fitness or health on its own.
Don't know your body fat? Estimate it with the body fat calculator, or see fat mass and fat mass index in the fat mass calculator.
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 1995;5(4):223–228 — the FFMI and normalized FFMI formulas.
- Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. Int J Obes 2002;26(7):953–960 — median FFMI 18.9 (men) and 15.4 (women), ages 18–34.