🚪 Cat Door & Flap Sizer
A cat flap is the one purchase where "roughly right" means a wrong-sized hole in your door. Measure your cat's widest point and shoulder height, and this sizer applies the brands' own measuring rules, matches you against real PetSafe and SureFlap models — and gives you a true-size cardboard template to test before anything gets cut.
Measure your cat
Measure a standing, awake cat against a wall. The two rules that matter, straight from the manufacturers' charts: flap width = widest point + 1–2″, flap height = tallest point + 1.5–2″. This sizer uses the safe (+2″) end and shows you the whole range.
Your minimum flap opening
Real models, checked against your cat
| Model | Flap opening | Door cut-out | Rating | Verdict |
|---|
Always compare against the flap opening — the hole your cat actually walks through — never the frame size on the box. Opening dimensions above are as published by PetSafe and SureFlap; cut-out sizes are shown only where the manufacturer publishes them (SureFlap). For PetSafe doors, the paper template in the box is the cutting authority.
📦 The cardboard test — true-size template
Brand charts are a starting point; your cat is the final judge. Print this outline at 100% / Actual size, cut it out of a cardboard box, tape it to the door at your planned mounting height, prop it open — and watch your cat walk through for a day before any real cutting happens. It is the test every size guide recommends and nobody else prints for you.
Calibration line — after printing, this must measure exactly 4 in (10.16 cm). If it doesn't, your printer scaled the page: set scale to 100% / Actual size and reprint.
Big-cat openings can span two printed pages — that's fine: print, overlap the sheets so the outline edges meet, tape them together, then transfer to cardboard. The calibration line is your guarantee the size stayed true.
Why use this sizer?
You only cut the door once
A carrier that's too small goes back to the shop. A cat flap that's too small leaves a wrong-sized hole in a door you own. Two honest measurements now are the whole difference between a five-minute install and a replacement panel.
"Small" is not a size
One brand's small-cat flap opens 5.25 × 8.125″; another's opens 5.6 × 4.7″. Same shelf label, very different holes. This sizer skips the marketing names and matches your cat against the actual published openings of real models.
The cardboard test, ready to print
Every good size guide ends with "cut a test hole in cardboard first" — then leaves you to draw it. Here the recommended opening prints at true size with a calibration line, so the test takes minutes, not measuring-tape archaeology.
How to measure — the three numbers
| Measurement | How | What it's for |
|---|---|---|
| Widest point | Tape across the chest or hips — whichever is wider — on a standing cat. | Flap width = this + 1–2″. Width is the dimension that most often rules a model out. |
| Tallest point | Floor to the top of the shoulders/back, standing tall (treats held high help). | Flap height = this + 1.5–2″. The flap should clear the top of your cat's back by at least 1.5″, ideally 2″. |
| Belly line (optional) | Floor to the lowest point of the belly. For most cats it's about 4.7″. | Sets a comfortable step-over: the lip your cat steps across must stay well below this line. |
Questions people ask
Should I size by the flap opening or the frame?
Always the flap opening — the smallest usable open space your cat passes through. Frames and cut-outs are bigger by design: SureFlap's microchip cat door, for example, has a 142 × 120 mm opening inside a 165 × 171 mm cut-out. A flap that "measures big enough" by its frame can still be too small where it counts.
I'm between two sizes. Which way do I go?
Up. Every manufacturer's guidance says the same thing: when your cat's minimums land between two models, take the larger one — and prefer the smallest size that genuinely fits over anything grander, since an oversized flap is heavier for a cat to push and leaks more weather.
How high off the floor should the flap sit?
Low. The bottom lip your cat steps over should stay under about a third of their leg height — roughly 2″ is a typical step-over for cats, and cat-sized doors are usually installed with the bottom of the opening 2–4″ above the floor. The quick check: most cats' bellies clear the ground by about 4.7″, and the lip must stay comfortably below that line. In a multi-cat home, set the height by the smallest cat.
My kitten is tiny. Can I just buy the small flap now?
You can, but you'll be buying twice. Size for the adult your kitten will become — use their expected adult measurements (breed size class is a good guide) and let the cardboard test confirm it. A too-big flap is a training problem for a few weeks; a too-small one is a new door.
The sizer says no cat flap fits my cat. Now what?
That's an honest answer, not a broken calculator — width is usually the limiting dimension. The next step up is small-dog sizing: published size charts list larger pet doors such as PetSafe's Large (10.25 × 16.25″ flap, rated to 100 lb). Big, broad cats use dog doors all the time; just keep the step-over low and run the cardboard test at that size first.
Keep exploring
Cat Harness Size Checker — the other measure-twice purchase · Carrier & Crate Fit Checker — sizing for travel · Cat Home Safety Audit — check the rest of the house too · All KittyButts tools
Sizing rules and model dimensions are summarized from manufacturer measuring charts (PetSafe) and published size guides (SureFlap) read in November 2026. Every flap ships with its own paper cutting template — that template, not this page, is the final word before you cut. When in doubt, the cardboard test outranks any chart, including this one.