Size, and everything that follows from it — a pet door has a larger opening for big cats and small dogs, needs a bigger hole, and takes its own tunnel extenders rather than the cat flap ones.
The mechanism is the same; the dimensions change the installation.
The opening
One pet door we review states 178 × 170 mm (7.01 × 6.69 in). A standard cat flap is smaller, and that difference decides whether a large cat squeezes or steps through.
The hole
A pet door cannot go where a cat flap was. Enlarging the aperture is carpentry in a wooden door and a new pane in glass.
The parts
Tunnel extenders are not interchangeable between the two ranges. Check the model before ordering.
Which you need
Measure the animal at the shoulders and the chest, then compare with the published opening. Breed names are no help here.
This does not replace your device’s manual. Blade temperatures, locking mechanisms and cleaning procedures differ from model to model, and the manufacturer’s instructions take precedence over anything written here. Unplug a device before dismantling it, check a hot blade or an air nozzle against your own hand before it goes anywhere near an animal, and keep a mechanism that closes or rotates under supervision until you know how it behaves.
Last reviewed 2026-08-17