How to Choose Pet Door Sizes? | Measure First, Cut Once

Buying a pet door by weight alone is the most common sizing mistake. Dogs and cats come in different shapes — a tall, narrow whippet can weigh the same as a stocky corgi but need a completely different flap. The honest way to pick a size is to measure your pet and compare those numbers against the flap opening, not the advertised weight range. Getting this right saves you from cutting a hole twice or watching your pet refuse to use a too-small flap. PetSafe’s sizing guidance, for example, starts with two measurements: width at the chest or hips, and height from the top of the shoulders down to the chest or belly. Add 1–2 inches to each for comfortable passage, then match the result to the chart.

Taking the Two Measurements That Matter

The first number you need is your pet’s widest body width. Measure across the chest and again across the hips, and use the larger of the two. Do this while your pet stands naturally, and skip the fur — measure the body itself. The second number is height from the top of the shoulders, also called the withers, down to the bottom of the chest or belly. That’s the clearance your pet needs to step through without scraping its back. Never measure to the head; the shoulders set the height the flap must clear.

Add 1–2 inches to both numbers. That margin gives a nervous pet room to pass without brushing the flap edges, which is often the difference between a door that gets used and one that gets ignored. If your pet sits right between two sizes on the chart, pick the larger one — PetSafe’s own fit guidance points out that a pet near the top of a size range usually needs the next size up to stay comfortable.

Comparing Your Numbers to Flap Openings, Not Weight

Weight ratings are a rough proxy at best because body shape varies so much. A tall, narrow dog may need a bigger flap than its weight suggests, and a short, wide one may need more width than height. This is why the flap opening measurement matters more than the “up to 40 lb” label on the box. PetSafe’s Freedom Aluminum Pet Door line gives you a clear sense of how sizes break down across weight ranges.

Notice that the flap opening, not the weight range, is what your pet actually passes through. A 35-lb dog with a deep chest might need the Large opening even though its weight falls in the Medium range. Measure first, then compare your pet’s width and height plus clearance against these opening dimensions. For door-mounted inserts, also check the step-over rise — the frame bottom sits above the floor, and that height must suit your pet’s shoulder height or it becomes a genuine barrier.

Verifying the Fit Before You Cut

Once your numbers point to a size, confirm the fit before committing to a hole in your door. Cut a cardboard mock-up of the flap opening and tape it to the door at the correct height. Watch your pet walk through it a few times. A comfortable pet steps through without hesitation; a nervous one backs off at the sight of a tight opening. This test matters most when your pet is on the edge of a size range or tends to be cautious about new things. If the cardboard test shows your pet brushing the edges, go up a size — that’s far cheaper than cutting a second hole in a real door.

If your pet clears the mock opening easily, you’re ready to buy. Before you order, double-check the model’s frame dimensions and rough-cut opening against your actual door type and thickness, since frame sizes vary by SKU even within one product family. When you’re weighing your options, our tested roundup of dog doors for exterior doors compares top models side by side to help you narrow the field.

Balancing Comfort Against Security and Insulation

A larger flap is easier for your pet to use, but it also lets more air in and out and can make the door easier for an unfamiliar animal to push through. The right choice is the smallest flap that still clears your pet comfortably — not the biggest one available. If you have a senior pet or one with mobility issues, err toward the larger opening with a lower step-over, since the extra clearance matters more than a marginal bit of insulation. For most pets, though, the 1–2 inch margin is the sweet spot between comfortable passage and keeping the door’s seal as tight as possible.