What is your FFMI, and what does it tell you?

What is FFMI?

Fat-free mass index is your weight with the fat taken out, divided by your height in metres squared. The 1995 paper that popularised the number defines it exactly that way and then adds a small correction so that people of different heights can be compared with a 1.8 m man.^[1]

The correction is 6.3 times the gap between 1.80 m and your height, added to the raw value. The calculator above does both steps and shows you each.

What do the two published reference points say?

• 18.9: the median FFMI for men aged 18 to 34 in a Swiss sample of 5,635 apparently healthy adults, published in 2002. It is a general-population figure, not an athlete one, and the sample was not randomly chosen and body composition was estimated by bioelectrical impedance.^[2]
• 25.0: the limit that the normalized values of the 74 athletes who had not used steroids "extended up to" in a 1995 sample of 157 male athletes. Some of the 83 steroid users in that sample were above 30.^[1]

The same paper estimated that 20 Mr. America winners from 1939 to 1959, before steroids were common, had a mean of 25.4, so even its own data put a group of elite lifters just above its limit.^[1]

Is 25 a natural ceiling?

The authors did not claim that. They called their findings preliminary and suggested the index might serve as an initial screen for possible steroid abuse in athletic, medical or forensic settings.^[1] A screen is a reason to look closer, not a verdict on anyone, and this page does not use it as one.

A single 1995 sample of 74 nonusers cannot set a rule for everyone. It is one group, measured at one time, and the abstract does not say how use was confirmed, so the number is a data point, not a law.

Why can your result move by a lot?

• Body fat is the weak input. A home scale, a skinfold caliper and a scan can disagree by several points, so the calculator shows what happens if your entry is off by 3 points in either direction.
• FFMI counts everything that is not fat as muscle: bone, organs, water and glycogen. Two people with the same FFMI can look and perform very differently.
• The height correction was built around a 1.8 m reference. Results at the extremes of height deserve less trust.

Who is this not for?

It is for adults who want to understand a number they have seen quoted. It is not for anyone under 18, not a diagnosis, and not a way to judge another person's body or choices. If you are worried about your weight, your body fat or how you feel about your body, talk to a clinician.

Where next?

See what the evidence looks like for the topics people argue about on the evidence ladder, read what public figures have said themselves on the White Pill Index, or send us a claim to check on the claim line.