FFMI calculator
How much lean mass you carry for your height, from your height, weight and body fat percentage. Kouri and colleagues at McLean Hospital in Massachusetts published the height-adjusted version in 1995, in a study of 157 male athletes: among those not using anabolic steroids, it reached a limit of about 25. It measures what BMI cannot, because BMI cannot tell muscle from fat.
Reading the number
FFMI answers a question BMI cannot: how much of you is not fat. Two people at the same BMI can be four or five points apart on FFMI, and that gap is muscle.
| Normalised FFMI | Men, roughly | Women, roughly |
|---|---|---|
| Below average | Under 18 | Under 14 |
| Average, untrained | 18 to 20 | 14 to 16 |
| Visibly trained | 20 to 23 | 16 to 18 |
| Very well trained | 23 to 25 | 18 to 20 |
These bands are conventions from the literature rather than clinical cut points. Nobody has defined a healthy FFMI the way the WHO has defined a healthy BMI, so treat the table as orientation, not as a grade.
The ceiling everyone quotes
Kouri's 1995 paper compared steroid users with non users and found that the non users clustered below a normalised FFMI of about 25. That single finding became the internet's natural limit. It is worth knowing, and worth holding loosely: the sample was small, it was all male, and individual genetics vary more than one study can capture.
Where the error comes from
FFMI inherits every bit of error in your body fat estimate. If your body fat is two points out, your lean mass is out by roughly the same proportion of your weight, and FFMI moves with it. Measure body fat the same way each time and compare your own numbers rather than other people's.
Common questions
What is FFMI?
Fat free mass index is your lean mass divided by your height squared, the same shape as BMI but with the fat taken out first. Because it ignores fat, it is a far better measure of how much muscle someone carries than BMI is, which is why it is used in strength sports and in research on muscle dysmorphia.
What is a normal FFMI?
Untrained men usually land between about 18 and 20, and trained men between 20 and 23. Women typically run several points lower because they carry less lean mass at the same height. Kouri's 1995 study found that drug free athletes rarely exceeded about 25, which is where the often quoted natural ceiling comes from, though it was a small sample and not a hard limit.
Why normalise to 1.8 metres?
Raw FFMI favours shorter people, the same bias BMI has in reverse. The normalisation adds 6.1 times the difference between your height and 1.8 metres, so a 1.7 m person and a 1.9 m person with the same build get comparable numbers. This page shows both.
Do I need to know my body fat first?
Yes, and the answer is only as good as that input. Use the body fat calculator on this site to estimate it from a tape measure, and remember that a few percentage points of error in body fat moves FFMI by a few tenths.
Sources
Every figure above comes from one of these. Check them.
- Kouri EM, Pope HG, Katz DL, Oliva P (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine 5(4)
- Schutz Y, Kyle UU, Pichard C (2002). Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. International Journal of Obesity 26(7)
- Woolcott OO, Bergman RN (2018). Relative fat mass (RFM) as a new estimator of whole-body fat percentage. Scientific Reports 8, 10980
Your numbers, kept
The app estimates body fat three ways and remembers past readings, so you can see whether lean mass is actually moving.
Free, no account, and it works offline. 100,000+ downloads on Google Play.
This is not medical advice. Body Numbers is a calculator. It does not diagnose, treat or monitor any condition, and no number it produces should replace a conversation with a doctor or a dietitian. If a result worries you, or if you are pregnant, unwell, or under eighteen, speak to a health professional.
