Measure the dog, not the breed chart
Breed-based size guides are a rough starting point at best. What actually matters is your dog's own shoulder height and chest width, since those two measurements decide whether they can pass through a flap comfortably without ducking or squeezing. Stand your dog square, measure from the floor to the top of the shoulder blades, then measure the widest point of the chest just behind the front legs. Add roughly 5 centimetres of clearance to both figures; a door that's an exact match forces your dog to crouch every time, which most dogs eventually just refuse to do.
The four common size classes
Manufacturers typically split doors into small, medium, large and extra-large, but the cutoffs vary between brands, so treat these as a rough map rather than a rule. Small suits toy and small breeds under about 7kg. Medium covers most cocker-spaniel-to-border-collie sized dogs. Large fits labradors and similar builds. Extra-large is built for giant breeds like mastiffs or great danes, and these often need a taller flap height as much as a wider one, since a tall dog ducking under a low frame is just as awkward as a wide dog squeezing through a narrow one.
Weight limits matter for electronic doors
If you're buying a microchip or electronic pet door, check the stated weight or size limit on the model itself, not just the general size class, because these doors are engineered with a specific flap weight and motor strength in mind. A flap rated for a 15kg dog forced to handle a 35kg dog daily will wear out its hinge or motor faster than advertised. If your dog is still growing, size for their expected adult weight rather than their current size to avoid buying twice.
Multi-dog households with different sizes
When dogs in the same house are very different sizes, size the door to the largest dog and accept that the smallest dog will have extra clearance. It's rarely worth installing two separate doors unless there's a real reason, like keeping a puppy confined while an adult dog has full access. Some electronic doors let you register each dog's chip separately, which solves the access-control side even if the physical opening stays sized for the biggest dog in the house.
When in doubt, size up slightly
A pet door that's marginally larger than strictly necessary rarely causes problems; a door that's too tight causes hesitation, reluctance, and sometimes a dog that stops using it altogether and waits at the back door instead. If you're unsure between two sizes, the larger option is almost always the safer call, particularly for older or arthritic dogs who need more room to move through without twisting a stiff shoulder or hip to fit.
