For the best sizing figure at least 2 inches extra for your dog's collar. Here is a basic chart that will help you in determining the correct sizes:
Size:
12" necks 8-10"
14" necks 10-12"
16" necks 12-14"
18" necks 14-16"
20" necks 16-18"
22" necks 18-20"
24" necks 20-22"
This collar tapers to a more narrow width by the buckle.