What Size Dog Door Do I Need? | Fit It Once, Correctly

Bought a dog door that was pure guesswork? You can’t return a hole cut in your wall, so it pays to get the sizing right before the saw comes out. The measurement rules are simple: width follows the widest part of the body (chest or hips), and height follows the distance from the top of the back down to the lowest point of the chest or belly. Add 1–2 inches to both. A flap that tops out at least an inch above the dog’s back clears the withers so nothing snags as they move through.

If you prefer a door mounted in an existing sliding glass door, the fit rules shift a bit, and the step-over rise becomes the hidden number to watch.

Taking The Three Measurements That Matter

Stand the dog on a level floor and keep them still. You need three numbers, not just one.

• Width — measure the widest point, usually the chest or hips, and add 1–2 inches of clearance. A dog needs room to squeeze through without the flap dragging across their flanks on either side.
• Height — measure from the top of the shoulders (withers) down to the lowest body point, typically the bottom of the chest or belly, whichever sits closer to the ground. Add 1–2 inches so the top of the opening clears the dog’s back.
• Step-over rise — for any door with a bottom threshold, this is the height the dog steps over to get through. It should never exceed one-third of the dog’s height at the shoulders, otherwise puppies, seniors, and short-legged breeds will struggle.

The flap top needs to sit at least 1 inch above the dog’s back, not the head. Remembering that one detail avoids the classic head-based measurement error that leaves the flap catching on the withers.

Using A Manufacturer’s Size Chart

Most charts start with weight ranges but still expect you to confirm actual body dimensions. PetSafe’s chart offers a good baseline, and the company’s own instructions require measuring real width and height before you buy, not just weighing the dog.

Weight is a starting point only. A lean 70-pound greyhound and a stocky 70-pound bulldog share the same chart row but need very different flap dimensions. Always compare your dog’s actual chest width and back-to-belly height against the opening size, and pick the next size up when your dog lands close to a chart boundary.

Build A Cardboard Mock-Up Before You Cut

Cut a piece of cardboard to the opening dimensions you’re considering, tape it in the doorway at the height the real door would sit, and watch your dog pass through. If they hesitate, hunch down, or bump the edges, the opening is too small.

For dog doors for sliding glass doors, the mock-up matters even more because the rise is already fixed by the track. Lowe’s installation guidance says to measure the rise the dog steps over and add 1 inch to that number when figuring the comfortable opening. If your dog clears the existing track easily and the added inch keeps their chest clear, you’re set. Dogs with short legs or mobility issues may need a different mounting approach, because door-mounted units typically require a minimum 3-inch step-over and some seniors won’t manage it.

Common Sizing Mistakes And Their Fixes

Most returns happen for a handful of avoidable reasons. Measuring the head instead of the back tops the list, followed closely by sizing on weight alone. Width and height both matter because a door that’s too narrow makes the dog scrub their sides on every pass, and a door that’s too short forces them to duck. For wall-mounted installations, forgetting the step-over rise leads to a door a short-legged dog can’t enter without a leap. And skipping the mock-up test converts a correct measurement into a guess.

Add a little extra allowance for thick coats and natural movement on the width side, and remember the flap top needs that inch of clearance over the back. A doorway that fits the dog’s body in both dimensions, measured honestly, installs once and works for life. PetSafe’s official sizing chart states both the weight ranges and the opening dimensions, so it pairs well with your own tape-measure numbers.