The standard advice for litter boxes is one per cat, plus one extra — a formula behaviorists call the n+1 rule. Most cat parents assume this is about controlling odor between cleanings. It is not. The plus-one exists because a litter box is a resource, and in a multi-cat home, resources get guarded.
Why one box per cat is not enough
A cat using the bathroom is briefly vulnerable: head down, senses partly occupied, escape routes limited. In a home with only as many boxes as cats, a more confident cat can control access to the only box in a room, or simply occupy it, without ever throwing a punch. The other cat learns that the box itself is a risk, and starts holding it or going elsewhere instead. The extra box removes the bottleneck: there is always an alternative, so no single cat can control the only bathroom in the house. The same ambush logic applies in single-cat homes with children, dogs, or heavy foot traffic near the box.
The size problem no one mentions on the packaging
Most boxes sold in stores are sized for kittens, not the adult cats who end up using them for years. A box should be roughly one and a half times your cat's body length, measured from nose to the base of the tail, not including the tail itself. For an average adult cat around 18 inches long, that means a box closer to 27 inches, well beyond what most standard covered boxes offer. A cat that cannot turn around, dig, and position itself without touching the walls will either avoid the box or use it in a way that lands outside it, which reads as a behavior problem when it is really a measurement problem.
Depth and litter type
Aim for 2 to 3 inches of litter, deep enough to dig and bury waste without hitting plastic. On litter type, preference studies consistently point the same direction: unscented, fine-grained clumping litter is preferred over scented, coarse, or crystal alternatives. Fragrance is for the person doing the scooping, not the cat using the box, and a strong scent layered on top of ammonia can be enough reason for a cat to start avoiding the box altogether.
Why the lid is the problem, not the solution
Covered boxes look tidier and contain litter scatter, but that containment works against the cat. A hood traps odor close to the litter instead of letting it dissipate, so the box smells stronger to a cat's nose exactly when it is deciding whether the box is usable. Covered boxes also cut down visibility and reduce the number of ways in and out, which matters in a multi-cat or multi-pet home, where a covered box with one entrance is easy to guard or block.
Placement matters as much as the box itself
Location can undo a perfectly sized, perfectly filled box. Avoid putting boxes next to washing machines, dryers, furnaces, or anything that can switch on unexpectedly, since a startling noise mid-use is enough to create a lasting aversion to that spot. Avoid dead-end placements too, such as a small closet or a narrow bathroom corner with a single way in and out, since that setup recreates the same ambush risk the n+1 rule is meant to solve.
A quick checklist for auditing your current setup:
- Count boxes: number of cats in the home, plus one.
- Measure your cat nose to tail-base, multiply by 1.5, and compare that to the box's actual length.
- Fill with 2 to 3 inches of unscented, fine clumping litter.
- Remove hooded lids where possible, or switch to open-top boxes.
- Relocate any box near appliances or inside single-entry dead ends.
- Spread boxes across different rooms rather than lining them up in one location.
If your cat suddenly starts going outside the box after using it reliably for months or years, treat that as a medical question before a behavioral one. Pain, a urinary tract issue, and several other conditions can all show up first as a change in litter box habits. Book an appointment with your veterinarian to rule those out — this article is not veterinary advice.
