How the math works
Felineering mathRecommended box length = your cat's length (nose to base of tail) × 1.5, a common guideline from cat behavior specialists. Boxes needed = number of cats + 1. Each box's length is the longest side in its seller's listed dimensions.
LimitsListed dimensions are outside measurements, and hooded or tapered boxes have less floor space inside, so pick a box with some margin. High sides help diggers; low entries help kittens and seniors.
Questions
How big should a litter box be?
About 1.5 times your cat's length from nose to the base of the tail. For an 18 in cat that's 27 in, longer than most standard boxes.
How many litter boxes do I need?
One per cat plus one, in different places.
What if no box is long enough?
A large storage tote with an entry cut into one side is a common alternative for big cats.