Cat Door Sizes: Flap Opening, Cut-Out Hole and How High to Fit It
Three numbers matter for a cat door: the opening your cat walks through, the hole you cut in the door, wall or glass, and how high that hole sits. Here are typical sizes, real product figures and how to measure your cat.
A standard cat flap has an opening of about 14–16 cm wide by 12–16 cm high (5.5–6.3 × 4.7–6.3 in) and needs a cut-out of about 16–17 cm (6.5 in) square. The SureFlap Microchip Cat Flap, for example, opens 142 × 120 mm and needs a 165 × 171 mm hole. Large-cat flaps such as the Cat Mate 221 need a cut-out of about 204 × 221 mm (8 × 8.7 in). Fit the bottom of the opening at the cat's belly height, usually 10–15 cm (4–6 in) above the floor.
- Standard opening
- 14–16 cm (5.5–6.3 in)
- Standard cut-out
- ≈ 16.5 × 17 cm
- Large-cat cut-out
- ≈ 20 × 22 cm (8 × 8.7 in)
- Fitting height
- 10–15 cm (4–6 in)
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Cat door sizes: opening and cut-out
Every cat door has two sizes. The opening is the clear space inside the flap that the cat passes through. The cut-out is the hole you make in the door, which must be larger to take the frame and tunnel. Product boxes often list only the outer frame, which is larger again, so read the specification carefully.
| Type | Flap opening | Door cut-out | Suits |
|---|---|---|---|
| Kitten or small flap | About 12–14 cm (4.7–5.5 in) wide | About 15–16 cm (6 in) | Kittens and small cats |
| Standard cat flap | 14–16 × 12–16 cm (5.5–6.3 × 4.7–6.3 in) | About 16–17 cm (6.5 in) square | Most adult cats up to about 5–6 kg |
| Large cat flap | About 17–18 cm (6.7–7 in) | About 20–22 cm (8–8.7 in) | Large and heavy cats, Maine Coons, small dogs |
| Small dog door | About 18–20 cm (7–8 in) wide and taller | Depends on model | Extra-large cats, cats sharing with small dogs |
Real product sizes
| Measurement | SureFlap Microchip Cat Flap | Cat Mate 221 Large Cat Flap |
|---|---|---|
| Flap opening | 142 × 120 mm (5.6 × 4.7 in) | Sized for large cats |
| Door cut-out | 165 × 171 mm (6.5 × 6.7 in) | 204 × 221 mm (8 × 8.7 in) |
| Outer frame | 210 × 210 mm (8.3 × 8.3 in) | 235 × 252 mm (9.25 × 9.9 in) |
| Door thickness | Tunnel depth 70 mm (2.75 in) | Self-lining up to 50 mm (2 in) |
| Glass fitting | Round hole about 212 mm (8.3 in), up to 260 mm with adaptor | Check maker's instructions |
Always cut from the template supplied in the box. Retailers sometimes list cut-outs a few millimeters different from the maker, and versions of the same model change.
How to measure your cat for a cat door
- Measure the width across the widest part of the body, usually the shoulders on a lean cat and the belly on a heavy one.
- Measure the height from the bottom of the chest (where the front legs join) to the top of the shoulders.
- Measure the belly height: floor to the bottom of the chest with the cat standing. This sets how high the door goes.
- Add about 2.5 cm (1 in) to width and height. The opening should be at least that large. Cats dislike flaps that brush their sides, and long-haired cats look smaller than they are only until the fur compresses.
A typical adult domestic cat of 4–5 kg (9–11 lb) measures about 10–13 cm (4–5 in) across and fits a standard flap. Large breeds such as Maine Coons, Norwegian Forest cats and Ragdolls often weigh 6–9 kg or more and need a large-cat flap. If you have more than one cat, size for the biggest.
How high to fit a cat door
Fit the bottom of the opening at about the cat's belly height, usually 10–15 cm (4–6 in) above the floor on the inside. Lower than that and the cat has to duck; higher and it has to step up, which older cats and those with arthritis avoid.
- Different floor levels: if the outside ground is lower than the inside floor, add a step or a block outside so the step-up on that side is no more than about 15 cm (6 in).
- Door rails and panels: on panel doors, the flap must sit in a flat panel or a solid rail. Do not cut through the frame's stiles or a glass panel's edge; most doors have a solid bottom rail about 15–25 cm (6–10 in) tall.
- Fire doors and security doors: cutting a fire-rated door voids its rating. Fit the flap in a wall or a different door instead.
Fitting a cat flap in walls, windows and glass
- Interior and exterior doors: most cat flaps fit doors up to about 50–60 mm (2–2.4 in) thick; check the specification against your door. Door sizes and thicknesses are covered in the door size guide.
- Walls: wood-frame walls are typically about 11–15 cm (4.5–6 in) thick and masonry cavity walls 25–30 cm (10–12 in), so you need a tunnel extension that spans the full depth. Line the tunnel to seal the cavity.
- Glass: you cannot cut toughened (tempered) safety glass after it is made. Order a new pane from a glazier with the round hole already cut to the flap maker's size. Double-glazed units need a new sealed unit with the hole through both panes. See window sizes and door and window dimensions for standard pane sizes.
- Sliding patio doors: insert panels with a built-in cat flap fit into the track without cutting. Measure the track height; panels are made to fit ranges of door heights.
Locking, microchip and draft-proofing features
Size is not the only difference between flaps. Four-way locks (in only, out only, open, locked) are standard on most models. Microchip flaps read the cat's implanted chip and open only for registered cats, which keeps neighborhood cats out; they take the same size cut-out as a standard flap of similar size. Magnetic flaps open for a tag on the collar instead. Brush or magnetic seals around the flap cut drafts, which matters on an exterior door in a cold climate.
An interior cat door lets a cat reach its litter box in a laundry room or closet while the door stays shut, and keeps a dog out if the opening is smaller than the dog. Interior doors are usually 35 mm (1⅜ in) thick, which every standard flap fits. If you need to keep a dog out of a whole area instead, see pet gate sizes.
Cats that use a flap to come and go often like a high lookout near it. A cat tree beside a window gives the same vantage point to indoor cats.
Frequently asked questions
What size cat door do I need?
Measure your cat's widest point, usually across the shoulders or belly, and its height from the bottom of the chest to the top of the shoulders. A standard flap with an opening around 14 × 12 cm (5.5 × 4.7 in) suits most cats up to about 5–6 kg (11–13 lb). Large, heavy or long-haired cats, and Maine Coons, need a large-cat flap or a small dog door with an opening of 17–18 cm (about 7 in) or more.
Does the width of a cat's head decide the door size?
No. A cat's head is narrower than its body, and its whiskers are roughly as wide as its body, so the shoulders, chest or belly set the minimum. A cat that can just get its head through an opening may still have to squeeze its belly, and many will refuse to use a flap that touches their sides.
What is the minimum size for a cat door?
Flaps with openings of about 14 cm wide by 12 cm high (5.5 × 4.7 in) are the smallest made for adult cats. Anything tighter is suitable only for kittens and very small cats. If the cat's measured width is within 2 cm of the opening, go up a size.
Can I fit a cat flap in glass?
Yes, but only in a new or replacement pane, cut with a round hole by a glazier, because toughened glass cannot be cut once made. SureFlap's microchip flap needs a round hole of about 212 mm (8.3 in), and its mounting adaptor accepts holes up to 260 mm. Check the maker's instructions, since other flaps need different holes.
How thick a door can a cat flap fit?
Most cat flaps fit doors up to about 50–60 mm (2–2.4 in) thick without extra parts; the Cat Mate 221 is self-lining for doors up to 50 mm. Thicker doors and walls need a tunnel extension, usually sold in sections.