OPENING WIDTH: a little wider then the widest part of your pet.

OPENING HEIGHT: measure from the top of the shoulders to the chest just behind the front legs.

Pet Doors FAQ recommends a Cardboard Test to ensure correct size for your pet. Using the OPENING SIZE measurement in the charts, cut a hole in a piece of cardboard, and see if your pet can pass through easily.

Generally there are four size groups ranging from small to extra-large. Often the manufacturer is weak on identifying what size will work best for you. Almost all cats and small dogs less than 20 pounds will fit through the small doors. Medium doors will often work for dogs up to 40 pounds, and large and extra large will work for those over 40 pounds.

Consider not only the current size and weight of your pet, but what you expect it to be in 2 years. Obviously, picking a door sized for a puppy today could easily be too small as they grow up. Also consider if you are likely to get more pets in the future and what size they might be.

Ideally you should measure your largest pet (or estimate what size they will be when full grown). Measure how wide your pet is and use a pet door opening that is at least an inch wider.

If you have both small and large pets, usually the larger pet door should work, although small pets may not be willing or able to push open a larger, heavier door. They also don't want to get run over when the large dog jumps in or out of the door! You might consider a powered door that physically opens for your pets, although these are quite expense.