How big? - The most important decisions to make in choosing a pet door are the size of the opening for your pet ('flap dimension') and,  just as important, the 'rise' or height of the pet door above the ground level. 

We can give you general guidelines regarding the size of pet door you'll need.  But only you can make the final decision taking into account your personal preference in addition to the range of sizes of pets to be using the door, the age and agility levels of each, and your plans for new pets in future.

Here are some general guidelines: If your pet is a cat smaller than about 15 lbs or a dog smaller than about 8 lbs, most 'small' (around 5" x 7" but some are smaller) size flaps will work.  Use a 'medium' (around 8" x 11") to about 35lbs and a 'large' (about 10"x15") to about 80lbs. Use X large (varies a lot but 15"x20" and 14"x23" are two common ones) for anything bigger. For most panel pet doors, where you cannot adjust the rise as you can with a pet door in a door or wall, choose the rise that gets the top of the flap as close as possible to the top of the pets shoulder. Note that the new Thermo Panel III has a 'User Adjustable Rise' which gives you much greater flexibility on the rise dimension.

You have three things to measure:

  1. Measure flap width - Simply open a door the minimum width needed by the broadest pet to comfortably pass and measure that gap. We'd like bigger pets to have about an inch more than the absolute minimum.  Be sure the pet door you select  has a flap at least that wide.

 2. Determine flap height - We recommend that the top of the flap be placed at least as high as the top of the tallest pet's shoulder (the withers). So measure that height. For a panel pet door, be sure that the height of the flap plus the rise is at least equal to that height. For a door mount, wall mount or screen mount, plan to install the pet door at least high enough off the ground to get the top of the flap up to the top of the pet's shoulder.  (Mounting even higher may result in the flap lasting longer for certain flexible flaps).

  3. Measure the 'rise' - The height established above, minus the height of the flap equals the 'rise' or the height of the step over for the pet. See if the rise you are planning is convenient for the pet to step over by holding a board across the threshold and having the pets step over it.

For example, let's say your 50 lb setter manages an 8" opening and is 22 at the shoulder. You're considering the Thermo Panel II and start with the medium as it is 8" wide. The medium flap is 11" tall and the tallest rise for the medium is 5" for a total of 16".  That's 8" too short! So you move to a large. Now, the flap is 15" tall and the tallest rise is 7" for a total of 22".  You have him step over a 7" high board and find that it's no problem. Bingo!

Now, suppose you also have a basset that cannot manage more than a 3" rise.   Even though either pet could use a Large by himself, you'd need to get an X-Large to get enough flap height to  accommodate both. 

To verify your selection, simulate the opening as shown and call the dog through as shown in the graphic above right. You'll immediately see if it is appropriate.  

HOT TIP: You can use tape (duct tape, masking tape etc) instead of boards for a quick and easy way to simulate an opening!
Simulate Width and Height of Opening You Are Considering

When you have made these decisions, you can compare your results to the flap dimension and rise information that you find for each pet door.  Of course, when you are looking at pet doors for doors or walls, the rise won't be stated because you can choose how high or low to mount the pet door.  However, for pet panels for sliding glass doors the rise is specified and cannot be changed.

News Flash! - That used to be true. Not anymore!  The new Thermo Panel III includes a patented User Adjustable Rise which allows the customer to change the rise dimension at home. Helps avoid mistakes and useful if your needs change.

Suppose you can't find exactly the size you come up with here?  Well, find the closest you can and then simulate that opening as shown in our drawing above. Pets can be extremely agile and you may find that it's 'close enough' to be satisfactory.

Choosing a flap size that is a little too big is probably better in the long run than one that's a little too small. One important reason for this fact is that, when a flap is larger than absolutely necessary vertically, you'll get longer flap life...the stress on the flap will be less.  Also, the agile puppy may be a little less so as he gets older.
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