This Collar Simply Slips over your pup's head and will tighten if he/she pulls. Measure the widest part of your dogs head/neck. Remember if your dogs head is wider than it's neck, then that's what needs to be measured! This collar runs true to size. For instance a 19” collar will fit over a wood disc 19” in circumference with room to slip 2 fingers between the collar and the disc.