What is FFMI?
Fat-free mass index (FFMI) measures how much lean mass — muscle, bone, organs and water — you carry for your height. It works like BMI but ignores fat, so it separates a muscular person from someone who is simply heavy.
How FFMI is calculated
Lean mass = weight × (1 − body fat %)
FFMI = lean mass (kg) ÷ height (m)²
Normalized FFMI = FFMI + 6.1 × (1.80 − height in m)
The normalization comes from Kouri et al. (1995). It adjusts everyone to a height of 1.80 m, because taller people carry more lean mass per square metre and would otherwise score higher.
FFMI chart for men
| Normalized FFMI | Rule-of-thumb level |
|---|---|
| Under 18 | Below average |
| 18–20 | Average |
| 20–22 | Above average |
| 22–23 | Excellent |
| 23–25 | Superior (near the drug-free limit Kouri observed) |
| 25 and above | Above the drug-free maximum Kouri observed (25) |
The FFMI natural limit
Kouri and colleagues measured 157 male athletes, 83 of whom had never used steroids. The drug-free group averaged a normalized FFMI of about 21.8, and none exceeded 25.0; many steroid users did. That is where the widely quoted “natural limit of 25” comes from. Treat it as a reference, not a verdict: body-fat measurement error alone can move FFMI by a point.
FFMI for women
Women carry less lean mass on average, so typical FFMI values run about 3–4 points lower than men's. Research on female upper limits is limited, so we don't show categories for women.
Estimating body fat
DEXA scans are the most practical accurate option. Smart scales and skinfold calipers are convenient but can be off by several percent, which shifts FFMI. Use the same method each time and focus on the trend.
Frequently asked questions
What is a good FFMI?
For men, a normalized FFMI of 18–20 is typical of untrained people, 20–22 shows clear training, and 22–25 is advanced. Women typically score about 3–4 points lower. The categories are a rule of thumb, not medical standards.
Is an FFMI of 25 really the natural limit?
In Kouri et al. (1995), none of the 83 drug-free male athletes studied exceeded a normalized FFMI of 25.0, while many steroid users did. It is a useful reference point, not a hard rule: a small number of people, and measurement error in body fat, can push results slightly above it.
What is the difference between FFMI and normalized FFMI?
Taller people naturally carry more lean mass per square metre, so normalized FFMI adjusts everyone to a height of 1.80 m by adding 6.1 × (1.80 − your height in metres). Use normalized FFMI to compare people of different heights.
How accurate is my FFMI?
FFMI is only as accurate as your body-fat estimate. A 3% error in body fat changes FFMI by about 0.7 points for an average man, so a DEXA scan gives a more reliable result than a mirror estimate.
General fitness information, not medical advice. Results are estimates; check with a doctor or qualified coach before major changes to diet or training. Last reviewed 2026-09-27.