What size pet door should I get? Dog doors and cat doors can look small from a human point of view, but the door opening doesn’t have to be as big as the pet standing upright. The opening width should be a little wider than the widest part of your cat or dog. The height should be as tall as from the top of the shoulders to the chest just behind the front legs. The pet will duck their head to push the door open, and lift their legs to go through. If you have an older pet that has problems lifting their legs, you might want to go up a size. If you have small and large animals using the same door, you might also want to go up a size, to get the opening close enough to the floor for the little ones to get through. To make sure you’re getting the right size, cut out the opening size in a piece of cardboard and see if your pet can get through without problems. Measuring a dog’s height can be quite taxing for both dog and person. Call your dog, and when they’re standing next to you, mark the height of the shoulder by touching your own leg. This way you can measure on yourself. Much easier!