Take the two measurements properly
Everything here comes from two numbers, and both are easy to get slightly wrong. Shoulder height is measured with the animal standing square, from the floor to the top of the shoulder blade — not to the top of the head, which is higher and produces a flap larger than it needs to be. Widest body width is measured across whichever is wider, the shoulders or the hips, with the animal standing rather than lying. For a broad-chested dog it is usually the shoulders; for some cats and many older dogs it is the hips.
Measure the animal you have now, and then think about the animal you will have in a year. A puppy grows; a door cut for a puppy gets recut. If the dog is not yet grown, size for a realistic adult figure rather than the current one, and accept a flap that is temporarily oversized.
Step-over is the number that decides whether the door gets used
The step-over, sometimes called the rise, is the height from the floor to the bottom of the flap. It exists because a flap that reaches the floor cannot seal and cannot be mounted in the bottom rail of a door. The trade is that every inch of step-over is an inch the animal has to lift over, and the same door that a young dog vaults through is one an older or short-legged animal simply declines.
The check used here is that the step-over should not exceed about a third of the shoulder height of the shortest animal using the door. That is a working rule rather than a standard, and it is deliberately conservative because the failure mode — a pet that refuses to use the door — wastes the entire installation. Where the door has to be mounted higher than that for structural reasons, a shallow step or ramp on both sides fixes it, and a ramp on only one side fixes half of it.
Flap size, rough opening and the manufacturer template
The flap opening is what the animal passes through. The rough opening is the hole you cut, and it is bigger by the frame on each side. The 1.5 inch per side default here is a common figure and it is not a specification: every manufacturer publishes a template with the exact cut-out for their product, and that template is the number to cut to. Use the calculator to decide which size of door to buy, then cut to the template that comes in the box.
| Mounting | What the install involves | Main risk |
|---|---|---|
| Door panel | Cut through the leaf, frame both faces | Cutting into a hollow-core door with no blocking |
| Through a wall | Tunnel kit, framing, flashing, insulation, seal both faces | Wiring, plumbing or structure in the bay |
| Glass or sliding door | Insert panel in the track, or a new pane made with the hole | Trying to cut tempered glass, which destroys it |
| Screen door | Cut and frame the screen | Assuming it seals or secures anything |
Through a wall is a building job, not a pet job
A wall installation is the tidiest result and much the biggest piece of work. The opening crosses the interior finish, the framing, the insulation, the sheathing and the cladding, and each of those needs handling. Check the bay for wiring and plumbing before any cutting — a stud finder plus a small inspection hole beats optimism. If the opening needs a stud cut, that is a framing question with a header, not something to improvise. The stud wall calculator covers layout and the header and beam load calculator is the right tool if a structural member is involved.
Then there is water. Any penetration through cladding has to shed water, which means flashing above the opening lapped correctly with the weather barrier, not a bead of sealant. The house wrap calculator covers the barrier side of that detail. Done badly, a pet door is a permanent leak in a wall cavity you cannot see into.
What a hole in the envelope costs you
A pet door is a deliberate air leak. Even a well made one with a magnetic closure and a double flap is a worse insulator than the door it sits in, and a cheap single flap in a windy location is noticeably worse than that. If the door opens onto a heated space this is a real running cost, and it argues for the smallest flap that genuinely fits the animal rather than the next size up for comfort.
Security is the other consideration and it is not paranoia: a large flap is a large hole, and the ones sized for big dogs are large enough to be a genuine entry point. Doors with a lockable slide-in panel exist for that reason, and are worth having whether or not you use them nightly.
What is on the other side
The last question is the one people answer after installing rather than before. A pet door means unsupervised access in both directions at any hour, and it means whatever is outside can find the hole. That covers fencing and gate latches, pool access, the neighbour's dog, and wildlife that will absolutely come through a flap if there is food on the other side. If the door opens into a run rather than an open yard, the dog run size calculator sizes that side, and the fence post calculator handles the perimeter. For an older animal where the step-over rather than the fit is the sticking point, the senior pet care guide covers the wider picture of mobility changes worth raising with a vet.
Questions people ask
What size pet door do I need for my dog?
Measure the shoulder height with the dog standing square, and the widest point across the shoulders or hips. The flap opening wants to be about two inches taller than the shoulder height once the step-over is subtracted, and about an inch wider than the body on each side. For a 20 inch dog with a 9 inch body width and a 4 inch step-over that comes out around 11 by 18 inches. Pet doors are sold in size bands rather than exact dimensions, so take the calculated figures and pick the smallest listed size that meets or exceeds both, then cut to that product's own template.
How high off the floor should a pet door be?
The bottom of the flap should be as low as the door construction allows, and no higher than roughly a third of the shoulder height of the shortest animal using it. Four to six inches is common for a medium dog. Too high is the most frequent reason a correctly sized door goes unused, particularly with older animals, short-legged breeds and cats, and it is a problem that gets worse over the life of the pet rather than better. If the mounting position forces a high step-over, add a shallow step or ramp on both sides.
Can I put a pet door in a hollow-core door?
Sometimes, but check first. Hollow-core doors are two skins over a cardboard grid with solid blocking only at the edges and around the lock, so a large cut-out can leave the flap frame with nothing structural to bear on. Small flaps low in the door often work; large dog doors frequently do not. The options are to fill the void around the opening with solid blocking before mounting the frame, or to fit the door into a solid-core leaf or through the wall instead. A wall installation avoids the question entirely and is a bigger job.
Will a pet door make the house colder?
Yes, to a degree that depends on the size, the quality and the exposure. A flap is a worse insulator than the door around it and it leaks air at the edges, particularly on a windy elevation. Double flaps, magnetic closures and a well-fitted frame all help, and so does keeping the flap no larger than the animal actually needs. If the loss matters, the honest fixes are a smaller flap, a better product, a location out of the prevailing wind, or a door into an unheated porch or garage rather than straight into living space.
How do I stop other animals coming in?
Physically, a flap sized for your dog is a flap sized for anything of similar dimensions, and raccoons and cats in particular will use one that is unlocked. Selective doors exist that unlock from a collar tag or a microchip and only open for the registered animal, which is the practical answer where wildlife is a problem. A manual lockout panel handles the overnight case. Beyond hardware, the most effective single change is removing whatever is drawing animals to the door, which is usually food left accessible on the inside.