Once you have decided to buy a pet door, your first consideration will be its size. Pet doors are not ‘one size fits all’ and the dimensions will vary as per the size of your pet. Well, we just made it a lot easier for you! Choose the most suitable size of pet door as per the breed of your furry friend.
In case you have more than one pet, make sure that the step over height is low for the smaller pet, while the height of the flap should allow the bigger(in size) pet to get through as well. It goes without saying that this is only for reference purposes as the pet sizes can vary intra-breed. You can actually try before you buy, by using a cardboard cut out of the same dimensions as the pet door.
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Source: enduraflap