Lean Body Mass Calculator (FFMI)
Calculate your body fat percentage and FFMI score using your height, weight and circumference measurements.
Enter all measurements in centimetres (cm) or kilograms (kg).
Not Weight, Muscle: Measure Your Real Fitness Level with FFMI
Body Mass Index (BMI) does not distinguish fat from muscle — a strong athlete can be labelled "obese" as a result. FFMI (Fat-Free Mass Index) removes body fat from the equation and measures only the density of muscle tissue. Used in sports science to define the natural muscle mass ceiling, it reflects your training progress far more accurately.
What Is FFMI?
FFMI is the normalised ratio of fat-free mass to height. It is calculated in the following steps:
- The body fat percentage (%) is estimated using the US Navy circumference formula.
- Fat mass (kg) is subtracted to find fat-free mass (FFM).
- Raw FFMI = FFM ÷ height(m)² calculates the index.
- Normalized FFMI = Raw FFMI + 6.1 × (1.8 − height_m) corrects for height differences.
How to Use This Tool
Required Measurements
- Height (cm) and weight (kg)
- Neck circumference (cm); at the narrowest point of the neck
- Waist circumference (cm); at navel level
- Hip circumference (cm): women only, at the widest point of the hips
How to Measure Correctly
Use a flexible tape measure. Measure neck circumference just below the thyroid cartilage (Adam's apple), and the waist at the midpoint of the navel. Keep the tape firm but not tight; take the measurement after exhaling.
FFMI Category Table — Male
| Normalized FFMI | Category |
|---|---|
| < 17 | Below Average |
| 17–18 | Average |
| 18–20 | Above Average |
| 20–22 | Good |
| 22–23 | Impressive |
| 23–26 | Outstanding (Natural Limit) |
| > 26 | Suspected Enhanced |
FFMI Category Table. Female
| Normalized FFMI | Category |
|---|---|
| < 13 | Below Average |
| 13–15 | Average |
| 15–17 | Above Average |
| 17–19 | Athletic |
| > 19 | Outstanding |
This tool is for informational purposes only. Body fat percentage is estimated via circumference measurements and may deviate ±3–5% compared to clinical methods such as DEXA or hydrodensitometry.
How to Interpret Your Results
Normalized FFMI adjusts your height to a reference of 1.80 m, so being short or tall does not distort the result. The difference between raw and normalized FFMI is purely a height effect. For tracking training progress, measure monthly under the same conditions (same time, fasted).
Natural Muscle Mass Ceiling
Sports science literature suggests the upper FFMI attainable without performance-enhancing substances is approximately 25–26 for men and 18–19 for women (Kouri et al., 1995). Scores above these values have been associated with steroid use, though some researchers argue the natural ceiling may be higher with exceptional genetics.
Tips for Accurate Results
- Measure in the morning, fasted; post-meal and post-workout measurements can give different results.
- Take circumference measurements at least twice and use the average.
- For individuals with very high muscle mass, the US Navy formula may overestimate body fat; a DEXA scan gives more accurate results in this case.
- Evaluate FFMI alongside BMI: low BMI + low FFMI may indicate insufficient body mass; high BMI + high FFMI points to muscle mass.