https://www.youtube.com/watch?v=1LvHF7Kf1fg&feature=youtu.be
Measuring for puppy collars
Using a flexible tape measure*, measure the circumference of your dog's neck where the collar is normally worn. The tape measure should fit comfortably around the neck without being too lapse or too tight – it should be a snug but a comfortable fit. The measurement you get is the "neck size" to use when ordering collars. Please choose the EXACT CIRCUMFERENCE of your dog's neck NOT the total length of the collar you require.
(* please note that if you don't have a flexible tape measure you can use a piece of string/ribbon or similar to measure your dog's neck and then measure this against a tape/ruler)