A comfortable neck measurement of 12" would be a 12" - 14" collar.
A comfortable neck measurement of 15.5" would be a 16" - 18" collar.
These collars are measured from the end of the buckle to the first hole and to the last hole. The adjustment from the first hole to the last is 2 inches. There are five holes. To measure for your dog, use a cloth tape measure or a piece of string and measure snugly, but not tightly around your dog's neck. This will be the small length of the collar. This leaves up to 2 inches to lengthen the collar to make it fit comfortably.