How to use this FFMI calculator
- Enter body weight and height in the selected units.
- Enter a body-fat estimate as a percentage.
- Select Calculate to see unadjusted FFMI, estimated fat-free and fat mass, and the separately labeled height-adjusted figure.
The formula and method
Fat-free mass (kg) = body weight (kg) × (1 − body fat percentage ÷ 100). FFMI = fat-free mass (kg) ÷ height (m) squared.
The separate height-adjusted figure uses the original Kouri convention: adjusted FFMI = FFMI + 6.3 × (1.80 − height in metres). At 1.80 m the adjustment is zero; above that height it subtracts from the unadjusted value.
Fat-free mass includes water, bone, organs and other tissues as well as muscle. The calculation is only as useful as the input body-fat estimate and should not be interpreted as a direct muscle measurement.
Worked example
An 80 kg individual, 180 cm tall (1.80 m) with 20% body fat: FFM = 80 × 0.80 = 64 kg. Unadjusted FFMI = 64 / (1.80)² = 64 / 3.24 = 19.75 kg/m². Because height is exactly 1.80 m, the normalized adjustment is 6.3 × (1.80 - 1.80) = 0, giving a normalized FFMI of 19.75 kg/m².
Understanding the estimate
- A change in the body-fat input changes fat-free mass and FFMI even if weight and height stay the same.
- Hydration and differences between body-composition methods can affect comparisons.
- FFMI does not establish an individual natural ceiling or prove whether someone uses performance-enhancing drugs.
Frequently asked questions
Why are there two FFMI values?
The first is fat-free mass divided by height squared. The second adds the original 6.3 × (1.80 − height) adjustment; it is a convention rather than a second measurement.
Is fat-free mass the same as muscle mass?
No. It includes all estimated mass other than fat, including water, bone and organs.
Can I compare results from different body-fat methods?
Use caution. A different method can change the fat-free mass estimate, making an apparent FFMI change partly a measurement difference.