What FFMI measures
Fat-free mass index is your lean body mass divided by your height squared. It works like BMI, but it leaves fat out, so it tells you how muscular you are for your height. Two people can have the same BMI and very different FFMI scores.
The formula
Lean mass is your weight multiplied by (1 minus body fat as a decimal). FFMI is lean mass in kilograms divided by height in meters squared. Normalized FFMI adds 6.1 times (1.8 minus your height in meters), which adjusts the score to a height of 1.8 m (about 5 ft 11 in) so that tall and short people can be compared fairly.
What your score means
| Normalized FFMI (men) | Meaning |
|---|---|
| Under 18 | Below average muscle |
| 18 to 20 | Average |
| 20 to 22 | Above average, clearly trained |
| 22 to 25 | Very muscular |
| Over 25 | Rare without years of training and strong genetics |
These bands describe men. Women's scores typically run about 3 to 4 points lower for the same level of training. A 1995 study of male athletes found that those who had not used steroids rarely scored above 25, which is why 25 is often quoted as a rough natural ceiling. It is a guideline from one study, not a hard limit.
The result is only as good as your body fat figure. If your body fat estimate is off by 3 points, your FFMI will be off by roughly half a point to a full point.
Common questions
What is a good FFMI?
For men, 20 to 22 shows a clearly trained physique and 22 to 25 is very muscular. For women, subtract about 3 to 4 points from those bands.
What is normalized FFMI?
It is FFMI adjusted to a standard height of 1.8 meters. Taller people score slightly lower on plain FFMI and shorter people slightly higher, so the adjustment makes scores comparable.
Is an FFMI over 25 possible naturally?
It is uncommon. One well-known study found that few steroid-free athletes exceeded 25, but some people do, and the score depends heavily on an accurate body fat measurement.
Is FFMI better than BMI?
For people who lift, yes. BMI counts muscle and fat together, so muscular people can be labeled overweight. FFMI only looks at lean mass.