FFMI is a way to measure your muscle mass relative to your height. If you know your body fat percentage, then this formula works out how much actual muscle mass you have. In other words, it's a BMI for muscle, which doesn't penalize you for being muscular.
The Calculation
First, calculate your lean body mass (total weight, without the fat):
Lean Mass = Weight × (1 - Body Fat % / 100)
Next, divide it by your height squared (same as BMI, but with the lean body mass):
FFMI = Lean Mass (kg) / Height² (m)
And lastly, adjust the result to account for height (to make it comparable between short and tall people):
Adjusted FFMI = FFMI + 6.3 × (1.8 - Height)
The 6.3 multiplier corrects for the fact that the taller you are, the smaller your FFMI will be. This adjustment puts everyone on an equal footing, at 1.8m (5'11").
Example
Here's a calculation for someone who is 180 cm tall, and weighs 80 kg, with 15% body fat:
- Height: 180 cm (1.80 m)
- Weight: 80 kg
- Body Fat: 15%
Step 1 - Lean Mass:
Lean Mass = 80 × (1 - 15/100) = 80 × 0.85 = 68 kg
Step 2 - Basic FFMI:
FFMI = 68 / (1.80)² = 68 / 3.24 = 21.0
Step 3 - Adjusted FFMI:
Adjusted FFMI = 21.0 + 6.3 × (1.8 - 1.80) = 21.0 + 0 = 21.0
The result of 21 falls into the 'Above Average' category. A reasonable level of achievement, likely through regular weight training.
FFMI Categories
| Category | Men | Women |
|---|---|---|
| Below Average | < 18 | < 13 |
| Average | 18-20 | 13-15 |
| Above Average | 20-22 | 15-17 |
| Excellent | 22-23 | 17-18 |
| Superior | 23-26 | 18-21 |
| Likely Enhanced | > 26 | > 21 |
Where Does the Natural Limit Come From?
Researchers took the average FFMI scores of Mr. America winners before the use of steroids became widespread (i.e. pre-1959). The average was 25.4, with the highest at 27.
The highest value of 25 was adopted as the maximum amount of muscle mass possible without steroids. Someone with an FFMI significantly above 25 was either a (genetic) freak of nature, or had been taking steroids.