It doesn't really work that way... you could weigh 140 lbs and have 35% or you could weigh 180 lbs and have 35% body fat. Body fat is a seperate thing from weight. You could be super muscular and weigh 150 lbs and have 25% body fat... you could be "skinny fat" and have 25% BF at 110 lbs. It measures the amount of lean tissue you have vs. the amount of fat. Your body composition can be different at different weights.