To measure your dog's girth, wrap the tape around your dog's body, going underneath the widest part of his rib cage (as shown in the graphic below). To get an accurate result, ensure that your dog is standing up while you position the flexible tape an inch or two behind your dog's front legs where a dog's chest is at its broadest point. Check that the tape is not too tight so that there is room for movement without being too loose. Record your dog's size · measure the length of your dog's back from the neck to the beginning of the tail. Check which length is best suitable for your dog.