It is 1 & 1/2 inches and tapers towards the buckle. Measure your dog's neck and figure 2-4" wider for the collar. The larger the size the closer to 3-4 extra inches you need to add. For smaller sizes 2" should be fine. 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"