FFMI values reported in studies
What researchers have measured in groups of athletes. These are observations of particular groups, not categories to sort yourself into.
| Who was measured | What the study reported |
|---|---|
| 74 male athletes who had not used anabolic steroidsKouri et al. 1995 | Normalized FFMI up to 25.0 |
| 83 male athletes who had used anabolic steroidsKouri et al. 1995 | Many above 25.0, some above 30 |
| 20 Mr. America winners from 1939 to 1959Kouri et al. 1995 | Mean 25.4, estimated by the authors |
| 235 college football players, by DXATrexler et al. 2017 | Mean 23.7. 26.4% above 25, highest 31.7 |
| 209 male college athletes in 10 sports, by DXACurrier et al. 2019 | Mean 22.8. 97.5th percentile 28.3 |
| 372 female college athletes, by DXAHarty et al. 2019 | Mean 18.8. 97.5th percentile 23.9 |
| 266 female college athletes, by DXABlue et al. 2019 | Mean 16.9. Range 13.3 to 25.5 |
The 1995 study used the height correction on this page. The later studies measured fat-free mass by DXA. Trexler and Currier adjusted for height with regressions of their own, Harty found that adjusting made no significant difference, and Blue reported FFMI without a height adjustment, so these figures are not directly comparable with a normalized FFMI from this calculator.
How it works
Fat-free mass index is body mass index with the fat taken out: fat-free mass in kilograms divided by the square of height in meters. It describes how much lean mass someone carries for their height.
The calculator does three things. It multiplies your weight by one minus your body fat to get fat-free mass. It divides that by height squared to get FFMI. Then it adds the height correction from the 1995 paper by Kouri, Pope, Katz and Oliva, 6.1 × (1.8 − height in meters), which adjusts the index to the height of a 1.80 m man, about 5 ft 11 in. Taller people get a small subtraction and shorter people a small addition.
Everything rests on the body fat figure. Fat-free mass is what is left after fat is subtracted, so an error in body fat becomes an error in FFMI. The table under the result shows where the index would be with body fat 3 points higher or lower than you entered.
The formula
fat-free mass
= kg × (1 − body fat ÷ 100)
FFMI
= fat-free mass ÷ height²
normalized FFMI
= FFMI + 6.1 × (1.8 − height)
mass in kg, height in meters
kg = lb ÷ 2.20462
m = inches × 0.0254The abstract of the 1995 paper prints the correction as 6.3 × (1.80 − height). The formula as the same research group has written it since has 6.1, and so does this page. The two versions differ by 0.02 for every 10 cm of height away from 1.80 m.
Worked example
A person who weighs 175 lb, is 5 ft 8 in tall and has 12% body fat.
- Weight: 175 ÷ 2.20462 = 79.38 kg. Height: 68 inches × 2.54 = 172.72 cm, which is 1.7272 m.
- Fat-free mass: 79.38 × (1 − 0.12) = 69.85 kg, which is 154.0 lb.
- FFMI: 69.85 ÷ 1.7272² = 69.85 ÷ 2.9832 = 23.42.
- Normalized: 23.42 + 6.1 × (1.8 − 1.7272) = 23.42 + 0.44 = 23.86.
If the body fat figure were 9% the normalized FFMI would be 24.7, and at 15% it would be 23.1.
What the 1995 study found
Kouri and colleagues calculated FFMI for 157 male athletes: 83 who had used anabolic-androgenic steroids and 74 who had not. Among the men who had not, normalized FFMI extended up to 25.0, which the authors called a well-defined limit. Many of the steroid users were above 25.0 and some were above 30. The authors also estimated the index for 20 Mr. America winners from 1939 to 1959, which they call the presteroid era, and got a mean of 25.4.
They wrote that the findings must be regarded as preliminary, and that FFMI may be a useful initial measure to screen for possible steroid use. An initial screening measure is a first look at a group, and this page does not use it to say anything about one person.
What the number cannot tell you
- The 1995 study measured only men. Later studies of female college athletes are in the table above.
- The 25.0 figure comes from 74 men. A later paper describes them as relatively lean (12.5% body fat on average), with at least two years of weight training, and of unclear athletic level.
- Larger groups have gone higher. Of 235 college football players measured by DXA, 26.4% had a height-adjusted FFMI above 25 by that study’s own adjustment, and 23.0% by the Kouri correction used on this page. The study did no drug testing of its own, and notes that NCAA athletes are subject to random, year-round testing.
- The result is only as good as the body fat figure behind it. The Navy tape method, for example, had a standard error of 3.52 points of body fat in the men it was built on, as the body fat calculator explains.
- A calculator cannot tell whether anyone has used steroids, and this one does not try.
Questions
What is FFMI?
Fat-free mass index: fat-free mass in kilograms divided by height in meters squared. It works like body mass index, but counts only the part of body weight that is not fat. For someone of 175 lb, 5 ft 8 in and 12% body fat it is 23.4.
How do I calculate FFMI?
Multiply weight in kg by one minus body fat as a fraction to get fat-free mass, then divide by height in meters squared. At 175 lb (79.38 kg), 12% body fat and 5 ft 8 in (1.7272 m): 79.38 × 0.88 = 69.85 kg, and 69.85 ÷ 2.9832 = 23.42.
What is normalized FFMI?
FFMI adjusted to a height of 1.80 m, using the correction in the 1995 paper by Kouri and colleagues: add 6.1 × (1.8 − height in meters). At 5 ft 8 in that adds 0.44, taking 23.42 to 23.86. The reason given for it is that fat-free mass divided by height squared still tends to be higher in taller people, who are also broader. In a 2017 study of 235 college football players, the correction moved values by 0.41 on average.
Is an FFMI of 25 the natural limit?
That figure comes from one study. In 1995 Kouri and colleagues found that normalized FFMI among 74 male athletes who had not used steroids went up to 25.0, and they called their findings preliminary. A 2017 study of 235 college football players, measured by DXA, found 26.4% above 25 after height adjustment. Neither study, and no calculator, can say what is possible for one person.
What FFMI have studies found in women?
The 1995 study did not include women. Two 2019 studies of female college athletes, both using DXA, reported a mean FFMI of 18.8 in 372 athletes, with a 97.5th percentile of 23.9, and a mean of 16.9 in 266 athletes, with values from 13.3 to 25.5.
How accurate is FFMI?
The arithmetic is exact. The inputs are not. Body fat is the weak point: for the 175 lb example, moving body fat from 9% to 15% moves the normalized FFMI from 24.7 to 23.1. A tape measure estimate of body fat can be off by more than that, as the body fat calculator explains.
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Sources
- Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine. 1995;5(4):223-228.
- Baggish AL, Weiner RB, Kanayama G, Hudson JI, Lu MT, Hoffmann U, Pope HG Jr. Cardiovascular toxicity of illicit anabolic-androgenic steroid use. Circulation. 2017;135(21):1991-2002. Table 1 note: the fat-free mass index formula with 6.1 × (1.8 − height).
- Trexler ET, Smith-Ryan AE, Blue MNM, et al. Fat-free mass index in NCAA Division I and II collegiate American football players. Journal of Strength and Conditioning Research. 2017;31(10):2719-2727.
- Currier BS, Harty PS, Zabriskie HA, Stecker RA, Moon JM, Jagim AR, Kerksick CM. Fat-free mass index in a diverse sample of male collegiate athletes. Journal of Strength and Conditioning Research. 2019;33(6):1474-1479.
- Harty PS, Zabriskie HA, Stecker RA, Currier BS, Moon JM, Jagim AR, Kerksick CM. Upper and lower thresholds of fat-free mass index in a large cohort of female collegiate athletes. Journal of Sports Sciences. 2019;37(20):2381-2388.
- Blue MNM, Hirsch KR, Pihoker AA, Trexler ET, Smith-Ryan AE. Normative fat-free mass index values for a diverse sample of collegiate female athletes. Journal of Sports Sciences. 2019;37(15):1741-1745.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Report No. 84-11. Naval Health Research Center, San Diego; 1984. DTIC accession ADA143890.
- National Institute of Standards and Technology. Guide for the Use of the International System of Units (SI), Special Publication 811, Appendix B.9: 1 inch = 2.54 cm and 1 lb = 0.4535924 kg.