Fat-Free Mass Index Calculator
The Fat-Free Mass Index (FFMI) describes how much lean (fat-free) mass you carry relative to your height, much as BMI describes total mass relative to height. It is popular for gauging muscularity independent of body fat. A normalised version adjusts FFMI to a reference height of 1.8 metres so people of different heights can be compared. Enter your weight, body fat percentage and height to compute your fat-free mass, FFMI and normalised FFMI.
FFMI formula
Fat-free mass = weight * (1 - body fat% / 100)
FFMI = fat-free mass / height(m)^2
Normalised FFMI = FFMI + 6.1 * (1.8 - height(m))
FFMI mirrors BMI but uses only lean mass. The normalisation term adjusts to a 1.8 m reference height, since taller people otherwise score lower for the same build.
Worked example
An 80 kg person at 15 percent body fat and 1.8 m tall: fat-free mass = 80 * 0.85 = 68 kg. FFMI = 68 / (1.8^2) = 68 / 3.24 = 20.99. Normalised FFMI = 20.99 + 6.1*(1.8 - 1.8) = 20.99.
Frequently asked questions
What is a typical FFMI?
For men, an FFMI around 18 to 20 is average, the low-to-mid 20s reflects substantial muscle, and around 25 is near the upper limit commonly seen in drug-free athletes. Women's values run a few points lower. Use it as a relative gauge, not a strict cutoff.
Why normalise FFMI?
Raw FFMI tends to be lower for taller people with the same build, because mass does not scale exactly with the square of height. The normalisation adjusts to a 1.8 metre reference so heights can be compared more fairly.
How accurate is FFMI?
FFMI is only as accurate as the body fat percentage you enter. Body fat estimates from calipers, bioimpedance or formulas all carry error, which flows through to FFMI. Use a consistent method to track change over time.
Sources
- U.S. National Institutes of Health (MedlinePlus): body composition measurement.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.