There are five holes punched in Hammond Leather Good dog collars and we always measure the collars from the collar fold to the middle hole on the collar to allow 1 inch of adjustment for tightening or loosening. See the picture below. So you'll have some wiggle room when you order your collar, and some room for growth, but please don't estimate; do take an accurate measurement. There are two ways to get the correct measurement for your dog's collar.
If your dog is not wearing a collar or you do not have its collar, you could wrap a string around the part of the dog's neck where the collar normally makes contact. Make sure the string is snug but not too tight, and mark it with your finger. Now measure from the end of the string to where you marked the string. This measurement is your dog's collar size at the third (middle) hole.
An even easier method to find your dog's collar size is to measure the collar your dog is currently wearing at the exact hole being used. This is the length of your new collar (measured from the post of the buckle to the third hole).