The Fat-Free Mass Index (FFMI) is like BMI, but for muscle. Instead of measuring total body mass against height, it measures your fat-free mass (your body weight minus fat) against your height. This makes it a much better tool for athletes and lifters to track their muscularity without being penalized for having low body fat. This calculator will give you your FFMI and your 'normalized' FFMI, which accounts for height variations, to show you where you stand.