Calculator Methodology
The GetFFMI calculator estimates lean body mass from weight and body fat percentage, then uses that lean body mass to calculate FFMI and normalized FFMI. The calculator runs in your browser and does not require an account.
Formulas Used
Lean body mass: weight × (1 − body fat percentage / 100)
FFMI: lean body mass in kilograms ÷ height in meters²
Normalized FFMI: FFMI + 6.1 × (1.8 − height in meters)
Important Limitations
FFMI depends heavily on body fat accuracy. If body fat percentage is underestimated, lean body mass and FFMI can look higher than they really are. The calculator is best used for context and progress tracking, not as a medical diagnosis or proof of natural status.
Male and female FFMI ranges are interpreted separately because average lean mass levels differ. A score that is moderate for a man may be very strong for a woman.
Privacy Reassurance
The calculator works directly in your browser. You do not need to create an account or submit personal information to calculate your FFMI.