Measure the length of the leather/webbing, including any quick release buckle (but excluding the chain), when the collar is set at the correct size.
Order a collar with this length in the middle of its size range for an adult dog, or one with growing room for a puppy.
If you don't have a collar that fits
Standard fitting: When the lead is taut, the collar just closes on the dog’s neck. The rings should meet, but there should be minimal constriction of the dog’s neck. This is the usual fitting for most dogs. Be aware that the collar opens a further 4” when the lead is loose.
Measurement 1: Place a tape measure around your dog’s neck and take a close-fitting measurement where the collar will sit. This is how big the collar needs to be when the chain is closed.
Measurement 2: Add 4” to Measurement 1. This is how big the collar will be when the chain is open.
Check that Measurement 1 does NOT slide over the dog’s head. If it does, you will need a tighter fit (see below)
Check that Measurement 2 DOES slide over the dog’s head. If not, you will need a quick release or buckle half check in order to get the collar on and off.
For an adult dog, order a collar with Measurement 1 in the middle of its size range. Allow growing room for a puppy.