“Density” is the number of hairs per unit area; eg. The hair density of a man between the ages of 30-50 is 250-300 per square centimeter on average. “Fullness”, on the other hand, is an objective criterion used to describe the “look” of one's hair, regardless of hair density. So much so that the hair of someone with less hair density may look fuller than someone with denser hair. Although hair density is the most important factor that determines fullness, other factors such as hair color, structure and thickness of the hair fiber are also very important.