Dog Door Size Calculator
Size a dog door flap and step-over height correctly from your dog's shoulder height and body width.
Floor to the top of the shoulder blades, standing.
Usually the shoulders, occasionally the hips.
Flap size
30 cm × 52 cm width × height
- Maximum step-over height
- 18 cm
- Riser below the flap
- 8 cm
- Measure the width at the widest point, which is the shoulders on most dogs and the hips on a few.
- The bottom of the flap should sit no more than 18 cm above the floor.
- A dog ducks slightly when going through, so the flap does not need to match full shoulder height.
- For a growing puppy, fit for the predicted adult size rather than cutting the hole twice.
- Consider a microchip-operated door if wildlife or neighboring cats are a concern.
How this is calculated
Two dimensions matter and one constraint. The flap must be wide enough to pass the dog's widest point and tall enough to clear its shoulders, and the step-over height at the bottom must be low enough to walk over rather than jump.
flap width = widest body width + 5 cm
flap height = (shoulder height × 0.85) + 5 cm
max step-over = shoulder height ÷ 3 Worked example
A dog 55 cm at the shoulder and 25 cm across needs a flap about 30 cm wide and 52 cm tall, with the bottom of the flap no more than 18 cm above the floor.
Measuring
- Shoulder height: floor to the top of the shoulder blades, dog standing square.
- Width: the widest point, which is the shoulders on most dogs. Some deep-chested and heavily-built breeds are widest at the hips, so check both.
- If several dogs will use it, size for the largest and check the step-over for the oldest.
Sources
- Flap dimensions and step-over heights are the fitting conventions used across common pet door ranges.
- Lowering the step-over height for arthritic dogs is standard mobility advice; we know of no trial that quantifies it.
Think about the dog in ten years
A step-over height that a young dog clears without noticing becomes a real obstacle to an arthritic one. Fitting the flap as low as the door construction allows costs nothing now and makes a difference later. If the dog will use it into old age, err low.
Placement
Put the door where the dog already goes, not where it is convenient to cut. Check for wiring, pipes and structural framing before cutting anything, and on a glass door use a purpose-made panel insert rather than attempting the glass yourself.
Common questions
What size dog door do I need?
The flap should be about 5 cm wider than your dog at its widest point, and tall enough to clear the shoulders allowing for the slight duck a dog makes going through. The bottom of the flap should sit no higher than a third of shoulder height.
How high should a dog door be off the ground?
No more than about a third of your dog’s shoulder height. Higher than that and the dog has to jump rather than step, which older or arthritic dogs will simply refuse to do.
Does the flap need to be as tall as my dog?
No. Dogs lower their heads slightly when going through, so a flap around 85% of shoulder height plus a small margin works. Making it full height weakens the door and lets more heat out.
What size dog door for a growing puppy?
Fit for the predicted adult size. Cutting a hole twice costs far more than fitting the right door once. Use the adult weight predictor to estimate the final size group.
Are dog doors a security risk?
A large flap is a genuine entry point, and insurers sometimes ask. Microchip and magnetic doors that only unlock for your own dog reduce the wildlife and neighboring-cat problem, and lockable panels let you close it at night.
My dog will not use the new door. What now?
Tape the flap fully open at first so it is just a hole, and call the dog through with food from the other side. Once that is easy, let the flap down a little at a time over several days. Most refusals come from the flap hitting the dog on the way through.