If you are taking a look at a Hale panel, then you would determine from down inside the bottom track then all the method approximately the leading track, stopping short of going inside the top track. Figure out how wide the flap needs to be. Open your sliding glass door broad enough for your animal to come through conveniently.
There's your minimum width. Do not purchase a flap that isn't a minimum of that wide. Determine the flap height and step-over measurements you need. Preferably, the top of the flap will be at least as high as the top of the animal at his 'withers' (top of his shoulders). That method, he does not need to crouch to use the family pet door.
That's just how much he has to step over. So, flap height plus step-over is the height of the top of the flap. Typically, the lower the step-over, the much better. Specifically when your family pet is older or arthritic, so you may prefer a flap 17" high with a 3" step-over to one with a 15" height and 5" step-over.
More flap and less step-over can also be handy if you've got a small animal that requires to show a larger one. A family pet door that includes a user-adjustable step-over makes it easier and provides more flexibility for the future. These are only offered with Endura Flap animal panels.
Many brands are offered in white, satin (aluminum color) or bronze (very dark brown). You can paint any of these to get a different color if you wish, but the simplest to paint is white. Inspect with your regional paint store for guide and paint suggestions. The material in which you are installing the insert does not affect the actual installation or performance of the animal panel so much as the measurements.
Make sure to examine these things prior to purchase. See if you have dual-pane glass. You probably do and, unless you remain in an Extremely temperate climate location, you'll desire to select a pet door that has dual-pane glass also. View Details can tell by putting a finger on both sides of your glass.