The collars are measured in a flat position, expect it to be a few centimeters smaller. Keep this in mind when specifying the measurement for the collar. The measurement you provide is for the collar in the tightened position (If you have chosen a half-choke, if you have purchased a fixed collar, you can disregard this!), and then I add for the choke part. The choke part is 10 cm unless otherwise specified.
Measurement instructions are available on the website in the main menu.
For Biothane collars, you don't need to add as much as for a sewn collar, as there is no lining taking up space against the dog's neck. 1-2 cm extra is usually sufficient.