How Many Litter Boxes per Cat? The N+1 Rule, Explained

The short answer: one litter box for every cat, plus one more. One cat gets two boxes. Two cats get three. Three cats get four. Vets and shelters call it the n+1 rule, and it is the single most common piece of advice for cats that start going outside the box.

Why One Extra Box?

Cats are fussy about toilets for good reasons. Some will not use a box another cat has just used. Some will not pee and poop in the same box. In a multi-cat home, one confident cat can sit near the only box and quietly stop the others from using it. An extra box in a different spot removes all of those problems at once.

Even single cats benefit. If one box is dirty, or the route to it is blocked by a closed door, there is always a backup.

Where to Put Them

The number only works if the boxes are in the right places. Purina's behavior guidance and most shelter setups agree on the basics:

• Separate locations. Two boxes side by side count as one big box to a cat. Put them in different rooms.
• One on every floor in a house with stairs, so older cats never have far to go.
• Quiet, not hidden. Away from the washing machine and loud appliances, but not in a dark corner that is hard to reach.
• Two ways out. A cat in a box with only one exit can feel trapped, especially with other pets around.
• Away from food and water. Cats do not like eating near their toilet, and neither would you.

How Big Should a Litter Box Be?

Bigger than most of the boxes sold as standard. Shelter guidance says a box should be at least one and a half times the length of your cat, nose to the base of the tail. If your cat has to crouch with its back end hanging over the edge, the box is too small. Extra large open pans and even plastic storage tubs with a cut entry are popular with cat behaviorists for this reason.

High sides help with cats that kick litter or pee standing up. For a second box on a budget, a large open pan does the job.

Open, Covered or Top Entry?

Many cats prefer an open box. Covers trap smell inside, which is nicer for us and worse for a cat's nose. If you want less litter on the floor, a top entry box is a good compromise for young, agile cats. It is a poor choice for kittens, seniors or cats with stiff joints.

Whatever you choose, keep about two inches of unscented litter in the box and scoop at least once a day. A full wash with warm soapy water every week or two keeps smells down.

Does an Automatic Litter Box Count?

It counts as one box. A self-cleaning box keeps that one box clean all day, which some cats love, but it does not replace the extra box in another room. Give your cat time to get used to it, with a regular box still available.

When It Is Not About the Box

A cat that suddenly pees outside the box, strains, or visits the box often with little result needs a vet, not a new litter box. Urinary problems can become an emergency, especially in male cats. Rule out health first, then look at the setup. For a full comparison of boxes, see our cat litter boxes guide.

Questions People Ask

Sources

• Purina: How Many Litter Boxes Per Cat
• HAWS Behavior Department: Litter Box Setup for Success (PDF)