How the math works
Felineering mathWeeks each piece lasts = the maker's stated life (for one cat) ÷ number of cats, when the maker states life per cat. Monthly cost = price per pack ÷ pieces per pack × (4.345 weeks per month ÷ weeks each piece lasts). Yearly cost = monthly cost × 12.
LimitsMaker-stated lives assume typical use; heavy use, hard water and large cats shorten them. Prices aren't stored here because they change daily: enter what you pay. Shipping, tax and subscriptions aren't included.
Questions
Why do I have to enter the price?
Retail prices change often, so the calculator uses the price you actually pay.
Does more cats always mean faster use?
For litter refills and drawer bags, roughly yes; makers state their life for one cat. Fountain filters are timed, so they don't scale with cats.
What about litter itself?
Litter use depends on box size, litter depth and your cats, and makers don't state it consistently, so we leave it out rather than guess.