A cat door interior installation solves a problem every cat owner knows: the cat needs access to the litter box, food station, or a favorite sleeping room, but you want that door closed. Propping doors open kills your HVAC zoning, lets noise travel, and gives the dog a free pass to the litter box buffet. An interior cat door costs $15 to $80, installs in under an hour, and permanently ends the door-cracking routine. This guide covers the types, the sizing math, where to put one, and exactly how to cut a hollow-core door without wrecking it.
Part of our Windows & Doors guide — explore the full guide →
Interior vs Exterior Cat Doors: Different Animals
Interior cat doors are simpler and cheaper than exterior pet doors because they don’t fight weather. There’s no need for magnetic weather seals, double flaps, insulated frames, or locking security panels. Many interior models skip the flap entirely — a plain open portal with a trim ring works fine between a hallway and a laundry room. If you’re looking for outside access, that’s a different product category with different requirements; see our guide to choosing an exterior door with a built-in pet door for that job.
Because the demands are lower, interior cat doors come in more playful designs: arched “mouse hole” portals, doors with brush strips that groom the cat on the way through, and lockable four-way flaps for when you need to confine the cat temporarily.
The Four Main Types
1. Flapless portals
A simple framed opening, usually oval or arched, with interlocking trim rings that clamp both sides of the door. Cats prefer these — many cats dislike pushing flaps. Typical price: $15-$30. The downside is zero sound or odor blocking, which matters for litter rooms.
2. Flap doors
A hinged clear or opaque flap in a plastic frame. Better odor control for litter areas. Look for a light flap with a soft magnet — heavy exterior-grade flaps discourage cats. Price: $20-$50. Four-way locking versions (in only, out only, open, locked) run $30-$60.
3. Grooming and brush portals
Portals lined with removable brush inserts that catch loose hair as the cat passes. Gimmicky but genuinely useful during shedding season. Around $25-$40.
4. Wall-mounted tunnels
When there’s no door in the right place, a wall pass-through kit tunnels through drywall between studs. Kits with telescoping tunnels ($40-$80) handle standard 4.5-inch interior wall thickness. Stay between studs, verify no wiring or plumbing with a stud finder and an inspection hole, and you’re fine.
Sizing: Measure the Cat, Not the Door
Cats need less room than you’d think, but the flap must sit at the right height. Two measurements matter:
- Width: measure across the cat’s widest point (usually the hips or chest). Add about an inch. Most cats fit a 6-inch-wide opening; large breeds like Maine Coons want 7.5 to 8 inches.
- Rise: measure from the floor to the cat’s belly. The bottom of the opening should sit at or slightly below this height — typically 4 to 6 inches off the floor. Cats step through; they don’t like jumping through a hole at knee height.
| Cat size | Weight | Minimum flap opening (W × H) |
|---|---|---|
| Small / kitten-adult | Under 9 lb | 5.5″ × 6″ |
| Average | 9-13 lb | 6″ × 7″ |
| Large | 13-18 lb | 7.5″ × 9″ |
| Maine Coon / XL | 18+ lb | 8″ × 10″ |
When in doubt, size up. A too-large opening costs nothing in performance indoors; a too-small one means the cat stops using it and you cut a bigger hole later.
The Best Use: Hiding the Litter Room
The number-one reason people install an interior cat door is banishing the litter box to a laundry room, closet, garage-adjacent mudroom, or basement. The setup works beautifully: the box stays out of sight, odors stay behind a closed door, and — critically — dogs stay out. A cat-sized flap physically excludes most dogs, and a microchip-activated flap ($90-$150, SureFlap is the standard) excludes even cat-sized dogs by reading your cat’s chip.
Three tips for litter-room doors specifically:
- Use a flap, not an open portal, if odor is the concern. The flap plus a good litter mat handles 90% of it.
- Put the door low. Senior and arthritic cats struggle with high step-overs.
- If the room is dark, cats don’t care — but you might. A $10 motion-sensor puck light near the box keeps cleanup pleasant.
DIY: Cutting a Hollow-Core Door
Most interior doors are hollow-core: two thin veneer skins over a cardboard honeycomb, with solid wood only around the perimeter. That makes them easy to cut but easy to splinter. Here’s the clean method:
- Leave the door hung. You can cut in place with the door held steady, or pull the hinge pins and lay it across sawhorses — flat is easier for beginners.
- Mark the opening. Use the template supplied with the cat door. Center it horizontally, bottom of the opening 4-6 inches above the floor line (remember the door itself sits about ¾ inch above the floor). Check the frame’s minimum edge distance — most need at least 3 inches from the door bottom.
- Tape the cut lines. Painter’s tape over the layout lines reduces veneer chipping. Re-mark on top of the tape.
- Drill starter holes. A ½-inch bit inside each corner gives your saw a place to start and produces rounded corners that resist cracking.
- Cut with a fine-tooth jigsaw blade. A 10-12 TPI down-cut or fine wood blade minimizes tear-out on the face you’re cutting from. Cut just inside the line. The honeycomb inside offers almost no resistance — let the saw do the work.
- Seal the raw edges. The cardboard core will show. The cat door’s two-piece frame covers it, but a bead of caulk or a strip of wood glue on the exposed core stiffens the edge before you clamp the frame on.
- Install the frame. Interior kits sandwich the door: one frame half on each side, screwed together through the door thickness. Most adjust for 1⅜-inch interior doors. Hand-tighten — overtightening dimples hollow-core skins.
Total time: 30-45 minutes. If the door is solid-core or a nicer panel door, the process is the same but slower cutting; on a paneled door, keep the opening within a single flat panel field or the frame won’t seat flush.
Renter and No-Cut Options
Can’t cut the door? Three workarounds:
- Swap the door. A basic hollow-core slab costs $40-$60 at a big-box store. Cut the new one, store the original, swap back when you move.
- Door strap / latch prop. A $10 adjustable strap holds the door open exactly cat-width. Zero installation, but you lose the closed-door benefits.
- Interior cat door panel for closets. Bifold and sliding closet doors can often be trimmed at the bottom rail by ¾-1 inch instead of cutting a hole — a gap cats happily use.
Mistakes to Avoid
- Mounting too high. The most common error. Cats want a low step-through, not a hurdle.
- Buying an exterior flap for interior use. Heavy insulated flaps with strong magnets frustrate cats and are overkill indoors.
- Cutting without a template. Freehand openings end up out of square and the trim frame won’t hide the gaps.
- Ignoring the dog problem. If a small dog can fit through the flap, the litter-room plan fails. Go microchip-selective or size the flap tight to the cat.
- Forgetting the door swing. A flap frame that projects ½ inch can clip nearby trim on doors that open against a wall. Check clearance first.
FAQ
Will my cat actually use a flap?
Most learn in a day or two. Tape the flap fully open for the first week, then half-open, then release it. Treats on both sides accelerate training. A minority of cats permanently refuse flaps — for them, a flapless portal is the fix.
Does an interior cat door hurt resale value?
A slab door is cheap to replace, so no. If you’re selling, spend $50 on a new blank door and patch nothing. Wall tunnels take a bit of drywall repair — two patches, mud, and paint.
Can I put a cat door in a door with glass?
Not through the glass without ordering a replacement pane with a pre-cut pet opening. Cut through the lower wood or composite rail only if it’s tall enough — otherwise choose another door. The same logic applies to a glass exterior door, where a glazier-cut insert is the standard route.
What about sound?
A flap blocks conversation-level sound reasonably well; an open portal blocks almost none. For a bedroom door, use a flap with a magnetic catch.
Bottom Line
An interior cat door is one of the highest-satisfaction-per-dollar upgrades a cat household can make. Spend $20-$40 on a properly sized flap or portal, mount it low, take your time with the jigsaw, and the litter box disappears behind a closed door for good. If the project has you evaluating the door itself and it’s a beat-up slab, it may be time to upgrade the whole thing — our door buying guide covers what slab and prehung doors cost before you cut.
