Generally, the most accurate way to determine your dog's collar size is by measuring your dog's neck. Measure at the narrowest point of your dog’s neck, just as tight or loose as you would like the new collar to fit.
The collar should be snug, without being so tight that it will cause discomfort or loose enough to slide over your dog’s head.
As a rule of thumb, a well-fitting collar should leave just enough room for you to slide two fingers between the collar and your dog’s neck.
Measuring your current collar
If you are satisfied with the fit of your dog’s current buckle collar, you can measure the collar itself to get a new collar that fits the same.
Measure from the buckle's center bar, where it connects to the collar, to the hole that you are currently using.