Why FFMI instead of BMI
The body mass index treats all weight the same, so a muscular athlete and a sedentary person of the same height and weight get the same BMI. The fat-free mass index (FFMI) removes fat from the equation: it divides your lean mass (muscle, bone, organs and water) by your height squared. It is a better way to track muscle gain, and it is widely used in sports science and in clinical nutrition to spot low muscle mass.
The formulas
- Fat-free mass = weight × (1 − body fat % ÷ 100)
- FFMI = fat-free mass (kg) ÷ height (m)²
- Normalized FFMI = FFMI + 6.1 × (1.8 − height in m)
The normalization, proposed by Kouri and colleagues in 1995, adjusts the result to a height of 1.80 m, because taller people naturally have a slightly lower FFMI for the same muscularity.
Worked example: a man of 180 lb, 5 ft 10 in, 15% body fat
- Convert: 180 lb = 81.6 kg and 70 in = 1.778 m.
- Fat-free mass: 81.6 × 0.85 = 69.4 kg (153 lb); fat mass 12.2 kg (27 lb).
- FFMI: 69.4 ÷ 1.778² = 22.0.
- Normalized FFMI: 22.0 + 6.1 × (1.8 − 1.778) = 22.1, above average and clearly muscular.
- His BMI is 25.8, “overweight” on the BMI chart, although most of that weight is muscle.
Typical ranges
| Normalized FFMI | Men | Women |
|---|---|---|
| Below average | under 17 | under 14 |
| Average | 17–19.9 | 14–16.9 |
| Muscular | 20–22.9 | 17–19.9 |
| Very muscular | 23–25 | 20–22 |
| Rare without drugs | above 25 | above 22 |
These bands are approximate. In Kouri’s study of 157 male athletes, the drug-free lifters stayed at or below a normalized FFMI of about 25, which is why that value is often quoted as a natural limit. A few genetically gifted people exceed it.
How body fat changes the result (same man)
| Body fat | Normalized FFMI | Lean mass |
|---|---|---|
| 10% | 23.4 | 73.5 kg |
| 15% | 22.1 | 69.4 kg |
| 20% | 20.8 | 65.3 kg |
| 25% | 19.5 | 61.2 kg |
The weak point: body fat
FFMI is only as good as the body fat percentage you enter. Bathroom scales with bioimpedance can be off by 3 to 5 points, which moves FFMI by about one point. Use the same method each time, in the same conditions (morning, before eating), and watch the trend rather than a single value. For a better estimate, a DEXA scan or skinfold measurements by a trained person are more reliable.