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 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 15lbs 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 Xlarge (varies a lot but 15"x20" and 14"x23" are two common ones) for anything bigger. For 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.

Now here is a more precise (and more complicated) discussion: Remember first that there is a very important subjective element here. Two different customers will have two different ideas about the flap size needed to accommodate a particular dog or cat. So we urge you to take a few minutes to measure as described below in addition to following these guidelines. We want you to be satisfied on your first try.

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. More really doesn't hurt.

2. Measure 'Rise' - What is the highest step that the shortest or most disabled pet can step over comfortably? For example, a taller dog, like a dobermann, can easily step over 6", 9" or possibly even more. While a shorter dog may only manage 3" or even less. A disabled or elderly pet may need a lower rise and you may need a lower rise when there is a step down on one side of the pet door (or, you can build a ramp). Actually place a board or a book in the doorway and call your pets over to judge this dimension.

3. 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 the minimum flap opening height is the difference between the maximum rise that you determined above and the top of the tallest pet at the withers. It can be more, of course, and we'd prefer to see more flap and less rise if possible. What's the impact of more flap height? Your flaps will last longer!

For Example: Let's say your 70 lb shepherd manages a 9" wide opening and easily steps over 9" (without jumping) and is 22" at the shoulder. Then the flap must be at least 13" tall (22"-9"=13"). Now, if you have already decided that you want the Quick Fit!, for example, you would choose the Large size (10"x15" flap) because the medium at 8"x11" is not wide enough and not tall enough. You'd be happy to get the 15" flap which is 2" taller than the absolute minimum you figured. And you'd choose the 7" rise to get the top of the flap to 22" (7"+15"=22").

Now, suppose you also have a basset that cannot manage more than a 3" rise. Your flap would then need to be 19" tall (22"-3"=19") to be most comfortable for both pets. Even though either pet could use a Large by himself, you'd need to get an XLarge 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.

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.

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.

We're sorry there are no absolutes here. It would be nice to be able to specify exactly the flap size you need. Unfortunately, the pet door world isn't like that. We just hope you'll give the matter some thought before ordering so that you'll be satisfied on your first try.