Last updated October 8, 2026 · Report an error
Fat-Free Mass Index (FFMI) expresses lean body mass relative to height, similar to how BMI expresses total weight relative to height. It is widely used in strength-sports research as a reference point for natural muscular development, using the normalized formula from Kouri et al. (1995). This is a fitness metric for general reference and educational purposes only, not a medical diagnostic tool.
How it works
Enter your weight, height, and body fat percentage. The tool calculates lean (fat-free) mass, then normalizes it for height using the published FFMI formula.
- Enter weight (kg).
- Enter height (cm).
- Enter body fat (%).
- Click Calculate to see your results.
Examples
An 80kg person at 175cm with 15% body fat
Lean mass = 68kg. FFMI ≈ 68 / 1.75² + 6.1 x (1.8 - 1.75) ≈ 22.5.
Who should use it
- Tracking lean mass development relative to height over a training program.
- Comparing muscularity across individuals of different heights.
Industry applications
- Strength sports and bodybuilding research
- Sports science and coaching
Advantages
- Normalizes lean mass for height, enabling fairer comparison across different heights.
- Based on a peer-reviewed, published formula.
Limitations
- Highly sensitive to the accuracy of the body fat percentage input.
- A fitness-research reference metric, not a medical diagnostic tool.
Common mistakes to avoid
- Using an inaccurate body fat percentage estimate, which significantly skews the FFMI result.
- Treating FFMI as a precise medical measurement rather than a fitness-research reference metric.
Best practices
- Use a consistent, reasonably accurate body fat measurement method (like skinfolds or a DEXA scan) for the most meaningful FFMI tracking over time.
Tips
- Re-test body fat percentage with the same method each time for the most meaningful FFMI trend over time.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
Published research reference ranges cluster around 18-22 for men and 15-18 for women without performance-enhancing drug use, per Kouri et al. (1995) — individual variation exists.
Kouri EM et al., "Fat-free mass index in users and nonusers of anabolic-androgenic steroids", Clinical Journal of Sport Medicine, 1995.
Accuracy depends heavily on how accurately body fat percentage was measured — small errors there significantly shift the FFMI result.