n + 1 Litter Boxes, and What the Litter Actually Costs

Why the spare box is not a spare

The plus-one exists because cats treat a soiled box as unavailable, and because in a multi-cat home one animal can quietly monopolise a box or block the route to it. A household with exactly as many boxes as cats has no slack at all: one dirty box on a Wednesday evening leaves a cat with a choice between an unacceptable box and the carpet, and the second option is the one that gets noticed.

The one-per-floor clause is separate and often more binding. A cat that has to descend two flights to reach a box will use it less often than one that does not, and an older cat with joint pain may simply stop making the trip. Two boxes side by side in a utility room satisfy the arithmetic and not the intent; the rule is about routes as much as counts, which is why the tool takes the larger of the two numbers rather than adding them.

Most boxes sold are too small, and the rule says by how much

One and a half body lengths is not a comfort target; it is the room a cat needs to turn, dig, cover and step out without touching a wall. For a 46 cm cat that is a box 69 cm long, and the standard open trays on a supermarket shelf run 40 to 55 cm. The gap is why a cat that has always used its box starts perching on the rim or eliminating just outside it — the box did not change, the cat grew.

The practical answer is often not a cat product. A plain plastic under-bed storage box of 70 by 45 centimetres costs a fraction of a designed litter tray, holds the depth a clumping litter needs and clears the rule with room to spare; cutting an entry in one end handles a cat that cannot step over a high wall. The rule is worth applying even when the shop does not sell anything that satisfies it.

The litter figure is a full-change model, and that is deliberate

The monthly volume assumes each box is emptied and refilled on the interval you set, with nothing recovered. Anyone who scoops daily and tops up will use less, sometimes far less — a clumping litter scooped well can go three or four weeks between full changes, which halves or quarters the figure. Read the number as the upper bound of a full-change routine and the lower bound as your own scooping discipline, because the calculator cannot know that part.

Two other things move it and neither is in the model. Litter type matters by a factor of two on volume for the same performance — a fine clumping clay compacts and lasts, a wood pellet swells and is discarded faster — and the box shape matters through depth: a shallow box refilled often uses more litter per month than a deep one changed rarely, for the same footprint. The footprint is what the tool computes; the routine is yours.

Frequently asked questions

How many litter boxes for two cats?

Three, or one per floor if the home has more than three floors. Two boxes for two cats is the arrangement that produces most of the litter-box problems a vet sees, and adding the third is the cheapest intervention available. Placing all three in one room does not count — the point is that a cat always has an unoccupied, clean box within easy reach.

Does a covered box count the same?

It counts in the arithmetic and behaves differently in practice. A hood holds odour in, which people like and cats do not; it also removes the escape route, so a cat that has been ambushed at a covered box may abandon it entirely. If you use covered boxes, the n+1 rule matters more rather than less, and at least one box in the house should be open.

Where should the boxes go?

Away from the food and water, away from a noisy appliance, and with more than one way in and out — a box in a cupboard with a single door is a trap from a cat's point of view. Spread them across the home rather than lining them up: three boxes in one corner satisfy the count and fail the purpose, because a cat blocked from that corner has no box at all.