Cat Doors & Flaps
Cat Doors: No-Cut Flaps vs. Glass-Cut Install
A no-cut flap screws onto a window frame, sliding door track, or thin wood door. A cut-in flap is built to be installed into an actual glass pane. Which one you need comes down entirely to what you're installing it into.
By Alex Rivera · Updated October 2026 · 4 min read
Written by a pet owner, not a content mill. For each pick I check the real product listing, materials and size, and recurring complaints in customer reviews where available, then name who it is not for. No brand pays me to rank picks. How I choose.
Quick answer
The right cat door depends on what you're installing it into, not on brand or price. If the door or window is already thin enough to screw a frame onto, a no-cut flap that mounts with screws and double-sided tape is the simpler, cheaper option and takes an afternoon. If the door is an actual pane of glass, a screw-on flap isn't an option at all, and the only way to fit a flap is to cut a hole into the glass itself, which means hiring a professional glazier rather than a DIY install.
- Check what you're installing into before picking a cat door: a screw-on flap and a glass-cut flap solve different problems and aren't interchangeable.
- A no-cut flap works on window frames, sliding door tracks, and thin wood doors, no hole-cutting needed.
- A flap built for an actual glass pane requires cutting into the glass, which means hiring a professional glazier, not a weekend DIY project.
- Look for a 4-way lock (free in/out, in only, out only, fully locked) so you can keep other animals out when needed.
A cat door only works if it matches what it's going into. A flap built to screw onto a thin wood panel or window frame won't seat or seal properly on a sheet of glass, and a flap built to be cut into glass is overkill (and a needless glazier bill) for a door that's already thin enough to mount a frame on directly.
Cat doors generally split into two installation methods. A no-cut flap that screws and tapes onto windows, sliding door tracks, and thin wood doors. And a round flap built to be cut into an actual glass pane, for a sliding glass door or window where nothing screws on because there's no frame to attach to. If your cat also needs a way to watch the outdoors once it's inside, see our cat window perches guide.
Which cat door fits your door
Look at what you're installing into before anything else. A window frame, a sliding door track, or a thin hollow wood door can take a flap that screws and tapes directly onto it, no cutting involved. An actual pane of glass, single or double-glazed, on a sliding glass door or a window, can't take a screw-on frame at all. The only way to fit a flap there is to cut a hole into the glass itself.
No-cut vs. glass-cut: how the two mechanisms differ
A no-cut flap mounts with included screws and double-sided tape onto an existing panel that's already thin enough, typically under an inch thick. It works on window frames, sliding door tracks, and thin wood doors, and the whole install is a same-afternoon job with a screwdriver.
A glass-cut flap is a different job entirely. A professional glazier cuts a round hole into the glass pane and the frame seats permanently into that hole, so the flap becomes part of the glass rather than something attached to a frame. It's typically made from UV-stable polycarbonate for outdoor use and makes sense specifically when the door or window in question is glass with nothing else to screw onto.
- Measure your cat's height and chest width before ordering, cat doors list a maximum size for the opening
- Check the panel or pane you're installing into first, that decides which type you need, not preference
- Budget for a glazier's time and cost if you're going the glass-cut route, it isn't a weekend DIY project
- Use a 4-way lock (free in/out, in only, out only, fully locked) to keep other animals out when you want the door closed
Neither flap makes a cat want to go outside alone. A cat door only controls access, it doesn't decide whether or when your cat actually chooses to use it.
Frequently asked questions
What is the best cat door?
It depends on what you're installing it into. A no-cut flap that screws onto a window frame, sliding door track, or thin wood door is the simpler, cheaper option whenever it applies. A flap built to be cut into an actual glass pane is the only option for a sliding glass door or window with no frame to screw onto.
Can I install a cat door without cutting a hole?
Yes, as long as you're installing into a window frame, a sliding door track, or a thin wood door. A flap like that screws and tapes on directly. Cutting is only necessary when the surface is an actual pane of glass with no frame to attach to.
Can I put a cat door in a glass door?
Yes, but it requires cutting a hole into the glass pane itself, which means hiring a professional glazier rather than doing it yourself. A screw-on flap won't seat or seal on bare glass.
How do I measure my cat for a cat door?
Measure your cat's height and chest width, and check that against the flap's opening size, since cat doors list a maximum size rather than a one-size-fits-all opening.
Will a cat door let other animals in?
Not if the door has a lock. Many cat doors include a 4-way switch (free in/out, in only, out only, fully locked), so check for that feature if keeping other animals out matters to you.