FFMI is simply how much muscle you’re carrying per unit of height. The problem with only using FFMI, though, is that as height increases, so does FFMI, regardless of whether or not someone has taken or is taking steroids.
Normalized FFMI takes this into account by normalizing people’s FFMI to the average height of the participants in the group being measured. In the largest and most commonly cited study on this topic, the average height of the participants was 5’11, so this is generally the height used when computing normalized FFMI (it’s also fairly close to the average height of American men—5’10).
If this all seems confusing, don’t worry—the key thing to remember is that normalized FFMI is slightly more accurate when comparing the FFMIs of different people, especially when they’re significantly taller or shorter than average.
Learn more:
Here’s How Much Muscle You Can Really Gain Naturally (with a Calculator)
6 Things You’ve Always Wanted to Know About Steroids
The “Good” and “Bad” Reasons to Take Steroids