These collars are measured from buckle to center-hole. The end product will have extra holes for adjustment in both directions. The best way to record your pet's measurement is to use their existing collar - measure the distance from buckle-end to their favorite hole. If you're buying a collar for the first time, you can take their measurement by using a piece of string: wrap it around your pet's neck, leaving enough slack to fit 2-3 fingers, and measure the distance from end to end. SIZE diagram