height of your dog
• To measure your dog's height, stand him next to a wall and mark a small mark with your pencil at the height of his tallest point on the back. (In most breeds this is the shoulder area although some wolfhounds and greyhounds have a "roach" that is higher)

height of the step-over or "RISE"
• To decide on the rise of your door, we need to consider how long your dog's legs are or how agile he is. A young, tall doberman can probably step over a rise 6-9" high with ease. A short, old basset hound might have difficulty with a 3" rise. If there is a setp on either side of the door, the step-up-step-down is also more difficult and the "rise" of the door should be made smaller to compensate.

  To test your pet's step-over height, place a 2x4 or 2x6 in the doorway and call your pet over it to test his ability to get over it with ease (without jumping).

A 24" doberman with a 14" basset who can only step over a 3" rise would look like this:
25" - 3" = 22" flap height.
The basset is fatter, so get a flap that is wide enough for him to use with ease.

Very small and very large pets: If there is a VERY LARGE disparity between the sizes of the pets such as a tiny terrier and a doberman, you may need to mount two different doors. (The weight of the flap may be too much for the tiny dog to push.) You may know this ahead of time and purchase two doors right off the bat, or you may want to try the large dog door and see if the terrier or cat will have the strength and tenacity to push it. Remember, however, that you want your pet to enjoy exiting through the door. Don't make it a chore.

The mounting height of the patio pet doors is pre-determined. You'll have to choose the one closest to your needs without the flexibility to change that measurement.