Here is a simplified guide: 1  Full head sew in weave: Hair 10 to 16 long. 2  4 ounces = A half head of extensions (to add body or length) 3  8 ounces = A full head of extensions.  6 - 8 ounces = A full head of extensions with a closure piece or frontal 1  piece. Full head sew in weave: Hair 18 to 24 long.