The definition of “body fat percentage” is built right into the term–it’s simply the percentage of your weight that’s comprised of fat. Thus, when you measure your body fat percentage, you’re determining how much of your weight is fat. For example: Say a person weighs 150 pounds and 15 of those pounds are fat. His body fat percentage would be 15 divided by 150, which, expressed as a percentage, is 10%.
This percentage changes if he gains or loses fat, and it also changes if he gains or loses muscle.