What Size Dog Door Do I Need?

A dog door is sized on two numbers: the width of your dog's shoulders and the height of its shoulders from the floor. Add a couple of inches to each and that is the minimum flap opening you need — not the frame, which is always bigger. The third decision, and the one people get wrong, is the step-over height: how far up the wall the bottom of the flap sits. Too high and an older or short-legged dog cannot use it; too low and you lose structural strength in the door. This page covers all three, plus install heights, multi-dog households and the security trade-offs.

How Do You Measure for a Dog Door?

Three measurements, all with the dog standing square on a flat floor.

Shoulder width. Measure straight across the widest part of the dog. For most dogs that is the shoulders; for barrel-chested breeds like Bulldogs and Staffies it may be the ribcage, and for some it is the hips. Measure the widest point, whichever it is. Add about 5 cm (2 inches).

Shoulder height. Floor to the top of the shoulder blades — the withers — not the top of the head. Dogs lower their heads slightly as they push through a flap, so head height is not the constraint. Add about 5 cm (2 inches).

Step-over height. This is not a measurement of the dog but a decision about installation, covered below.

Measure over the coat, and take each figure twice.

And for a puppy, measure the projected adult, not the dog in front of you. A dog door is a permanent hole in a wall or door — you fit it once. The general technique for holding a dog still and getting consistent numbers is in how to measure a dog for a harness.

Flap Size vs Frame Size

The distinction that causes most wrong purchases.

The flap opening is the clear space the dog actually passes through. This is the number that has to fit your dog.

The frame is the whole unit, including the surround. It is always larger — often by 5–8 cm in each direction.

The cut-out is the hole you make in the door or wall, which is between the two.

Listings vary in which number they lead with. Some advertise the frame, some the flap, some the cut-out. Find all three before buying: the flap must fit the dog, the cut-out must fit your door.

Where this bites. A "large" dog door with a 30 × 38 cm flap needs a cut-out closer to 35 × 43 cm, and a door with a narrow stile may not physically accommodate it. Measure your door before you measure the catalog.

Typical Dog Door Size Chart

Sizes are not standardised between manufacturers. Check the actual flap dimensions rather than the size name, and check them against your own two measurements plus the 5 cm allowance on each.

When in doubt, go up one size. A flap slightly larger than needed costs a little heat and a little security; one slightly too small is either unusable or teaches the dog to force its way through, which wears the flap and can catch the coat.

Getting the Step-Over Height Right

The decision that determines whether the door gets used at all.

The step-over is the distance from the floor to the bottom of the flap.

Rule of thumb: no higher than roughly one third of the dog's shoulder height. A dog with 60 cm shoulders can comfortably step over about 20 cm.

Too high and the dog has to jump or scramble. Young fit dogs manage; older, arthritic, short-legged and heavy dogs do not, and they will stop using the door — often years after it was fitted, as they age.

Too low and you weaken the bottom rail of a door, or leave the flap vulnerable to draughts, flooding and being pushed in.

In an external door, the practical minimum is usually around 8–10 cm from the floor to leave the bottom rail intact.

Plan for the dog getting older. A step-over that suits a five-year-old Labrador may defeat the same dog at twelve. If you are fitting a door once, fit it low.

And check with the actual dog before drilling. Mark the proposed opening on the door with tape, and see whether your dog will walk through the marked space without hesitating. Measure the dog itself the way you would for any other equipment — the technique is in how to measure a dog for a harness, and the crate size calculator will give you the shoulder figures if you have not taken them yet.

Where Can a Dog Door Go?

Not only in a door, and the options differ in cost and difficulty.

External door. The usual choice. Simplest to fit, but limited by the width of the stiles and rails, and it puts a permanent hole in the door.

Wall. More work — you need a tunnel section to pass through the wall thickness — but it frees up door placement entirely and is often the only option for very large dogs. Check for cables and pipes before cutting.

Sliding patio door. Panel inserts fit into the track without modifying anything, which makes them ideal for rented properties. They reduce the opening width of the door, and some compromise the lock.

Window. Possible with a panel insert, generally suitable only for small dogs and ground-floor windows.

Glass door. Requires a specialist to cut toughened glass, or the glass unit replacing. Expensive.

Whichever you choose, think about what is on the other side. A dog door opening onto a garden gate that gets left open is a gap in your containment, and that matters more than any sizing decision on this page.

Multi-Dog Households

Where sizing gets genuinely awkward.

Size the opening for the largest dog — everything must fit through.

Set the step-over for the smallest or the oldest. Those two constraints usually conflict.

A Labrador and a Dachshund together is the hard case: the Labrador needs a large flap, and the Dachshund needs a step-over of a few centimeters. A single door low enough for the Dachshund and wide enough for the Labrador leaves a very large hole.

Two doors is often the honest answer in that situation — one sized for each — rather than a compromise that suits neither.

Consider a magnetic or microchip door if one dog should have access and another should not, or if you want to exclude neighborhood cats and wildlife. These read a collar tag or the dog's own microchip and unlock only for the right animal.

And watch how they use it. Two dogs going through together, or one crowding the other, is a sign the flap is too small even if the measurements say otherwise.

Security, Weather and the Trade-Offs

A dog door is a hole in the building, and it is worth being clear-eyed about that.

Security. Any opening large enough for a big dog is large enough to be a concern. Mitigations: a lockable slide-in panel for nights and holidays, siting the door where it is not visible from the street, and microchip-activated doors that stay locked by default.

Heat loss. Single flaps leak heat badly. Double-flap and magnetic-seal designs are considerably better, and worth the extra on a door that stays in year-round.

Wind. A large flap in an exposed position will bang constantly. Magnetic closures and heavier flaps help.

Water. A low step-over in a spot that puddles will let water in. Check where the rain runs before choosing the location.

Wildlife. Cats, foxes and, in some places, less welcome visitors will use an unlocked dog door. A dog door is also worth thinking about alongside where the dog sleeps — a bed right beside a draughty flap is a poor combination, and dog bed size guide covers placement as well as sizing. Microchip doors solve this and are the main reason to pay for one.

And the training. Most dogs work a flap out within a few sessions with treats on the far side, though some need the flap taped open at first. A dog that is genuinely frightened of the flap needs the noise and the resistance reduced — a lighter flap, or the magnets removed while it learns.

Teaching a Dog to Use It

A correctly sized door that the dog refuses to go through is a hole in your wall and nothing more.

Start with the flap taped or propped open. Let the dog walk through an open hole first, with someone on the other side calling and rewarding. Most dogs cross an open gap without hesitation.

Then lower it gradually. Tape the flap half open for a day or two so the dog pushes through a partial obstruction rather than a closed one. Only then let it hang fully.

The noise is usually the problem, not the flap. Magnetic closures snap shut behind the dog with a bang, and a sound-sensitive dog will decide the door bit it. Remove the magnets while it learns and refit them later.

Reward from the far side, never push the dog through. A dog shoved through a flap once may not approach it again for weeks.

Keep sessions short and frequent — a couple of minutes, several times a day, is far better than one long attempt.

Older dogs learning late need more patience and a lower step-over than a young dog would tolerate. If a dog that has used the door for years suddenly stops, check whether it has become stiff rather than assuming it has become stubborn — the reasons are usually physical, and joint change is the most common of them.

Common Dog Door Sizing Mistakes

Buying on the frame size instead of the flap opening.

Measuring head height instead of shoulder height. Dogs duck through flaps.

Measuring the shoulders when the ribcage or hips are wider. Measure the widest point.

Setting the step-over too high. The single most common reason a fitted dog door stops being used, usually as the dog ages.

Sizing for a puppy. The hole is permanent; the puppy is not. Project the adult size, weighing monthly with how to weigh your dog at home and checking the result against healthy weight range for dogs by size.

Not checking the cut-out against the door. Some doors physically cannot take a large unit.

Forgetting about heat loss. A single flap in a much-used external door is a noticeable draught.

Ignoring what the door opens onto. A dog door into an unsecured garden is a containment problem, not a convenience.

Compromising in a multi-dog household when two doors would serve better.

Not checking with the dog before cutting. Tape the outline on the door and watch your dog walk through it.

FAQ

What size dog door do I need?

Measure your dog's widest point across the shoulders and its shoulder height from the floor, then add about 5 cm to each. That gives the minimum flap opening — check it against the flap dimensions, not the frame size.

Do I measure my dog's height to the head or shoulders?

To the shoulders, at the withers. Dogs lower their heads as they push through a flap, so head height is not the limiting measurement.

How high should a dog door be off the floor?

No more than about one third of your dog's shoulder height. A dog with 60 cm shoulders can comfortably step over around 20 cm. Set it lower if the dog is short-legged, heavy or likely to develop stiff joints.

Is the flap size the same as the frame size?

No. The frame is always larger than the flap opening, often by 5–8 cm in each direction, and the cut-out sits between the two. Buy on the flap dimensions and check the cut-out fits your door.

What size dog door for a Labrador?

Usually XL, with a flap around 38 × 43 cm. Check your own dog's shoulder width and height first, as Labradors vary considerably between lines.

Can I fit one dog door for a big dog and a small dog?

You can, but the compromise is awkward — the opening must suit the largest dog while the step-over must suit the smallest. In households with a big and a very small dog, two doors often works better than one.

Two Measurements and One Decision

Dog door sizing is shoulder width plus 5 cm, shoulder height plus 5 cm, checked against the flap opening rather than the frame — and then a separate decision about how high off the floor the flap sits. Get the step-over low enough that the dog can still use it in ten years' time, check the cut-out against your actual door before buying, and tape the outline up and watch the dog walk through it before you cut anything. If you are sizing other things at the same time, the crate method is in how to measure your dog for a crate.