If you do not have the old collar to measure, or maybe he has grown since then here is what most do. ( Now this is NOT for a sighthound..) Since this is buckle type collar, you can put it on without going over the head. I usually measure around the neck about a inch and a half down off the throat for a large breed. Then you add two inches. This allows the collar to be comfortable on the dog but fit nicely. Also- with leather youwant it pretty soft as sometimes they can rub the neck as new leather- just like a pair of new leather shoes would do with you. Lighter leather in collar is less likely to " bleed" the stain or color. However, I have found a black dye stays better than a dark brown. For the first few days, take the collar off for a hour or two to allow the skin to breathe. Some dogs are allergic to the dyes in the leather.. Take a look at the neck when you take it off looking for any signs of rubbing or leather dye reactions. A good leather collar will last for years- a poorly made one can be a problem. Also I roll mine in my hands for like 30 minutes to make it more " broken in" and plyable if it is a stiff collar. Also- you want double stitching at where the buckles meet etc for obvious reasons. Just like with any collar, check for wear once in a while.