The best method is to measure the dogs neck directly. Take a measuring tape and wrap it around your dogs neck just where you would like the collar to sit. No need to add extra room or pull tight, we will make it adjust comfortably from the dogs actual neck size.
You can also measure from a current collar your pet wears. If measuring from a leather collar, measure from the fold in the leather (by the buckle) to the hole that is most used and the best fit for your dog. Measure a nylon collar from the start of the material on one end to the end of the buckle on the other end. See the pictures below.