FFMI is a measurement of how much muscle you have per unit of height. Although imperfect, 
research shows FFMI is a reliable indicator of whether or not someone is taking or has taken steroids to build muscle. You can calculate your FFMI by dividing your fat-free mass (your total body weight minus the weight of your body that’s fat) in kilograms by your height in meters squared, or simply entering your height, weight, and body fat percentage into the Legion FFMI Calculator.