Free FFMI calculator · no signupYour mass.
Minus the fat.

What FFMI actually measures

Body mass index divides weight by height squared, so it cannot tell a heavily muscled lifter from someone carrying the same weight as fat. Fat-free mass index runs the same arithmetic on lean mass alone, which gives you a figure for how much non-fat tissue you carry relative to your frame.

The normalization adjusts toward a 1.8 m reference height. Skip it and tall people score low while short people score high for the same build, because lean mass does not scale with the square of height. The normalized figure is the one usually quoted, and it is the large number in the panel.

The body-fat problem

FFMI inherits every bit of error in your body-fat estimate, and that error usually runs larger than people expect. Two points of body fat shifts a typical FFMI by roughly half a point, which is enough to cross a band boundary.

Rough reliability of the common methods:

• DEXA is the practical reference standard, though scans vary between machines and hydration states.
• Calipers stay reasonably consistent when the same person measures each time. Better for tracking change than for absolute numbers.
• Bioelectrical impedance scales respond to hydration, food, and time of day. Treat a single reading with suspicion.
• Visual estimation feeds most calculators like this one and routinely misses by five points in either direction.

If your body-fat number is a guess, read the FFMI as a range. Enter 12% and 18%, look at both results, and you have a more honest picture than either figure alone.

Reading the bands

The comparison bands come from published samples of athletes and non-athletes, most prominently Kouri and colleagues' 1995 study of drug-free and steroid-using lifters. They describe those samples. They are not a diagnostic scale, and the female ranges rest on considerably less data than the male ones.

You will often see the claim that a normalized FFMI above about 25 indicates steroid use. That comes from the upper bound of one small 1995 sample of 157 men, and it has been repeated far past what it can carry. Genetics, training history, measurement error, and body-fat estimation each move the number by more than the margin the claim depends on. This page will not tell you whether someone is natural, and neither will any other FFMI calculator.

What it is useful for

Tracking yourself over time, mostly. Measure body fat the same way each time and FFMI change tells you whether a training block added lean mass or just weight, which scale weight alone cannot. That makes it a reasonable companion metric through a gaining phase.

It does a poor job of comparing two different people, since you would be comparing two different body-fat estimates made by two different methods. It also says nothing about what you can do with the tissue. For that, strength standards against a competitive field will tell you more.

None of this is medical advice, and FFMI is not a health screen. If you want body composition interpreted in a health context, that is a conversation for a clinician.