Measure around the dog’s head at the widest part the collar will need to slip over (1). This is usually under the jaw and over or right in front of the ears. These collars are fully adjustable but do not have a buckle so accurate measuring is important. Collar should be big enough to slip over the head when fully extended, but smaller than the smallest part of the neck (2) when fully contracted. If you have any concerns about sizing, please contact us before purchasing. We're here to help!