If you are using Knotty Boy, you can start dem locks with hair as short as 3 inches long. It is difficult, but not impossible, and the dreads actually turn out better in the end when started short. They will be very short dreads, kind of Buckwheat-style for a while, but that's supa cute if you ask us. However, the longer the hair is to start out with, the easier it will dread in the beginning. And without Knotty Boy, no less than 6 inches to start dreads - they won't hold together if it's any shorter.