If you don’t have one, here is a tape measure you can download and print: https://z5d9i3t2.stackpathcdn.com/wp-content/uploads/2020/11/measuringtape-A4.pdf
While your dog is standing – measure snugly around your dog’s neck where the collar would sit. Take this measurement and then add one inch for a small dog and two inches for a larger dog. This is the collar size for your dog. Remember your dog needs enough space for 2 fingers to fit between the collar and their neck for a comfy fit.