An XL litter box should measure at least 1.5 times your cat’s body length from nose to tail base, so an 18-inch cat needs roughly 27 inches of interior length.
For the full breakdown, see our best XL Litter Box guide.
Most boxes sold as “extra large” fall short of that mark, and the label means nothing on its own. There is no standardized XL size in the US, so a jumbo box from one brand can be smaller than a large box from another. The only number that matters is the one you get from measuring your own cat and multiplying by 1.5.
Veterinary guidance from the American Association of Feline Practitioners and the American Animal Hospital Association backs that formula, and both note that a lot of manufactured boxes are simply too small.
How Do You Measure a Cat for a Litter Box?
Measure from the tip of the nose to the base of the tail, then multiply by 1.5 to get your minimum interior length.
Some guidelines measure to the tip of the tail instead, which produces a longer number. Either version works — the tail-base method is the more common one, so start there and round up if your cat is borderline. What you cannot do is measure only the torso. A box that fits the body but not the tail base is too short.
Width matters just as much. The cat must be able to turn a full circle, dig, squat, and cover without its shoulders or hips brushing the walls. For most adult cats, a box under 17 inches wide will not allow that turn.
Here is what the formula produces for common cat sizes:
- 18-inch cat (nose to tail base): 27 inches minimum interior length
- 20-inch cat: 30 inches minimum
- 22-inch cat: 33 inches minimum
Maine Coons, Norwegian Forest Cats, and other large breeds frequently land in the 20- to 24-inch range, which pushes their minimum into the 30-inch-plus territory. That is beyond what most commercial boxes offer.
What Do Real Extra-Large Boxes Actually Measure?
Commercial boxes labeled XL usually run between 22 and 30 inches long, and not all of them clear the 1.5x bar.
The dimensions below are reported examples, not an industry standard. They show the range you will encounter:
| Box Type | Interior Dimensions | Best For |
|---|---|---|
| Large commercial box | 29.73 × 17.5 × 17.64 in | Cats up to 19 inches nose-to-tail |
| Large high-sided box | 24 × 18 in, 10 in walls | Litter scatterers, smaller adults |
| High-sided box, larger cut | 26.25 × 21.5 in, 10 in walls | Big cats that kick litter |
| Open-box style | 22.4 × 19.7 in | Average adults, easy access |
| Extra-large open container | 30 × 30 × 9 in | Large breeds needing floor space |
| Plastic latching storage box | 32.125 × 19.125 × 13.75 in | XL cats when converted |
Notice the gap. A 24-inch box works for a cat measuring 16 inches nose to tail base. It does not work for a 20-inch Maine Coon, whose minimum is 30 inches. That mismatch is exactly why so many owners end up converting a storage tub. A quick scan of tested extra-large options shows how wide the real-world spread runs if you would rather buy than build.
Why Do Cats Prefer Bigger Boxes?
Fifty centimeters is under 20 inches — a useful floor, not a target. The study’s test boxes were modest by XL standards, which tells you the preference for space starts well before you reach the 1.5x formula. Cats want room to move, and a cramped box creates stress that shows up as avoidance.
Uncovered boxes suit most cats better than hooded ones. A lid traps odor and blocks the cat’s view of its surroundings, which is a real problem for a cat that feels cornered. High sides without a roof give you scatter control without the trade-off.
Litter depth runs about 2 inches for everyday use, with up to 3 inches of digging material if your cat likes to burrow. Whatever depth you choose, the box still needs enough interior height for the cat to dig without hitting the bottom or sides.
How Should You Set Up a Large Box?
Pick the largest box that lets your cat complete the full routine — enter, turn, dig, eliminate, cover — without touching a wall, then place one more box than you have cats.
The one-per-cat-plus-one rule comes from veterinary behavior guidance, and it holds even in single-cat homes where a second box adds a choice of location. Distribute boxes in quiet, accessible spots rather than lining them up in one room.
If no commercial box is big enough, a plastic storage container is a legitimate answer. Cut an entry on one long side, then file or sand the cut edge smooth — a sharp rim is an injury risk. High sides reduce scatter; a low entry keeps the box usable for kittens, seniors, and cats with arthritis.
Owners of arthritic or mobility-limited cats should skip high walls entirely and look for a low-entry design instead. The same goes for kittens, who need to climb in without a struggle.
The Bottom Line On Sizing
Measure your cat nose to tail base, multiply by 1.5, and treat that number as the floor — not the goal. An 18-inch cat needs 27 inches; a 20-inch cat needs 30. If the box you find falls short, a storage tub gets you there for less money than most oversized commercial options.
Add roughly 2 to 3 inches of litter, keep the box uncovered, and remember the one-per-cat-plus-one count.
Common Questions
Is a 24-inch litter box big enough for a large cat?
It depends on the cat’s length, not its weight. A 24-inch box suits a cat measuring about 16 inches from nose to tail base. Larger breeds and long-bodied cats typically need 27 to 33 inches of interior length, which pushes them into storage-tub territory or purpose-built oversized boxes.
Do covered litter boxes work for big cats?
Most cats prefer uncovered boxes, and the preference is stronger for large cats because a hood removes usable interior height. A covered box also traps odor and limits visibility, which can make a cat feel trapped. High sides without a lid give you scatter control while keeping the box open.
How much litter should go in an extra-large box?
About 2 inches is standard for daily use, and up to 3 inches of digging material if your cat likes to burrow. A deep fill in a tall-sided box can leave too little interior height for the cat to dig freely, so check the remaining headroom before you pour.
Sources
- American Association of Feline Practitioners. “General Litter Box Considerations” Supports the 1.5x body-length guideline, litter depth, and box-per-cat count.
- Ohio State University Indoor Pet Initiative. “Litter Boxes” Supports uncovered box preference, turning room, and litter depth guidance.