FFMI Calculator

How an FFMI calculator works

FFMI stands for fat-free mass index. It’s your lean mass divided by your height squared, the same shape of sum as BMI but with the fat taken out first. This FFMI calculator needs three things: your weight, your height and your body fat percentage.

Take 85 kg at 15% body fat and 180 cm. Fat mass is 12.75 kg, so fat-free mass is 72.25 kg. Divide by 1.8 squared, which is 3.24, and the FFMI is 22.3. At exactly 1.8 m the height adjustment is zero, so the adjusted number is 22.3 too.

Why there are two numbers

Kouri and colleagues normalized every athlete to a 1.80 m man, so lifters of different heights could sit on one scale. The correction adds 6.1 for each metre you’re shorter than 1.8 and takes it off for each metre you’re taller. Keep the same 72.25 kg of lean mass but stand at 170 cm, and you get 25.0 raw and 25.6 adjusted. Here’s the pattern for one fixed amount of lean mass:

One wrinkle. The paper’s abstract gives the constant as 6.3, while the formula as reproduced in Jan Todd’s critique, and in most calculators, uses 6.1. We use 6.1. Between 160 and 200 cm the two differ by at most 0.04, which won’t change anything you’d act on.

Our view: to track your own progress, watch the raw FFMI. Your height isn’t changing, so the adjustment only adds a constant.

What the research says about FFMI

Any FFMI calculator that draws a natural limit is leaning on one paper, so here’s what’s actually in it. It’s Kouri, Pope, Katz and Oliva, in the Clinical Journal of Sport Medicine. They measured 157 male athletes: 83 steroid users and 74 nonusers. The height-normalized FFMI of the nonusers reached 25.0 and went no higher. 20 Mr. America winners from 1939 to 1959, before steroids were common, averaged 25.4. Many users went past 25, and some past 30. The authors called the findings preliminary and suggested FFMI as a first screen for steroid abuse.

That 25 has since been taken further than the paper took it. Jan Todd’s 2008 critique in Iron Game History says Pope’s body fat estimates for the old Mr. Americas were never published, and by her recalculation 15 of 22 of them, nearly 70%, scored above 26. She also quotes the authors themselves: one cannot definitively diagnose steroid use simply on the basis of the FFMI. Eric Helms, in Alan Aragon’s Research Review in 2014, looked at the same data and argued that even if some self-described nonusers had lied, a ceiling in the region of 25 would hold.

For everyday context there’s Schutz and colleagues, who measured 5,635 adults in Switzerland (2,986 men and 2,649 women) using bioelectrical impedance checked against DXA. Among 18 to 34 year olds the median FFMI was 18.9 for men and 15.4 for women. Those are the median lines on our scale.

Our view: treat 25 as a landmark, not a wall. It’s where one sample of drug-free men topped out. It isn’t a rule about any one person.

Where this FFMI calculator falls short

Everything hangs on your body fat number. In the example above, being three points off moves the adjusted FFMI by about 0.8. If you’re guessing your body fat, you’re guessing your FFMI. Our body fat percentage calculator gives you a starting estimate.

Fat-free mass isn’t muscle. It’s everything that isn’t fat, including bone, water and organs. Kouri’s athletes were all men, so we show the 25.0 line for men only, and the women’s median comes from a single Swiss sample.

And it’s one number. It doesn’t know your training age, your genetics or where you carry your muscle. If you’d rather put numbers on eating for muscle, our protein calculator is a better next stop than fiddling with this FFMI calculator.