How much water does an indoor cat need per day?
About 4 ounces of water per 5 pounds of body weight per day. For an average 10-pound cat that comes to roughly one cup a day. The number isn’t all from the bowl: wet food is at least 75 percent water, so canned-food cats get a big chunk through meals and drink less; kibble (6 to 10 percent water) makes the bowl do most of the work. Don’t chase the number with a measuring cup. Watch the trend over weeks instead. A real change in how much your cat drinks is what matters.
When to call a vet: if your cat suddenly drinks much more or much less than usual, see a vet, since changes in drinking can signal kidney disease, diabetes or an overactive thyroid.
