Calculate fat-free mass index (FFMI) from height, weight, and body fat percentage.
How It Works
How Fat-Free Mass Index Calculator Works
Body fat percentage is used to split your total weight into fat mass and fat-free mass, and that fat-free mass is then divided by height squared — the same structure as a BMI calculation, but using lean mass instead of total weight. A normalized version adjusts the figure to what it would be at a height of 1.8 m, which is what most FFMI reference charts are built around, making comparisons across different heights more meaningful.
Worked Example
See It In Action
At 1.75 m, 80 kg, and 15% body fat, fat-free mass comes to 68 kg, giving an FFMI of about 22.2 — in the "excellent" range on the reference scale.
FAQ
Frequently Asked Questions
Why does FFMI matter for people who lift weights?
It's often used as a rough natural-muscle-mass ceiling reference — very high FFMI values are uncommon without significant muscle-building history, which is why the scale flags unusually high results.
How accurate is my body fat percentage input?
The result is only as accurate as the body fat percentage you enter — home methods like calipers or bioelectrical scales carry a meaningful margin of error, so treat the FFMI result as an estimate that moves with your body fat accuracy.
