Free tool
FFMI Calculator
FFMI (Fat-Free Mass Index) relates your estimated lean mass to your height — a better signal for “am I actually building muscle” than weight or body fat percentage alone. Fill in the three fields below to calculate yours.
Your result
FFMI
21.4
Normalized FFMI
21.7
Fat-free mass
65.6 kg
Adjusted to a 5′11″ (1.8m) reference height, so people of different heights compare more fairly.
What these numbers usually mean
These ranges come from a small 1995 study of male athletes — they are not a natural limit, a diagnosis, or an exact dividing line. Sex, age, frame size, and the quality of your body-fat estimate all change the interpretation. Treat this as a rough reference, not a ruler.
The formula
FFMI = fat-free mass (kg) ÷ height (m)². Normalized FFMI adds a 6.1 × (1.8 − height in m) adjustment to compare people of different heights more fairly — it's this normalized version that most published calculators and studies actually use.
The hard part is the body fat percentage
FFMI depends entirely on the quality of the body fat percentage you feed it. A caliper, a bioelectrical scale, or a guess will each give you a meaningfully different answer — and that error carries straight through to your FFMI. Corpus estimates your body fat percentage from a check-in photo, using the same capture every time, and calculates normalized FFMI automatically at every check-in, no arithmetic required.