A cat weighing 4 kg or about 9 pounds needs to drink a minimum of 150 mL of water per day to meet his fluid needs. This is equal to 0.634 cups; a bit over half a cup.
A wet food diet will supply the majority and perhaps all of a domestic cat’s fluid needs. You can see this if you feed your cat a wet food diet. Some domestic cats almost never drink water or commercially prepared milk when on a wet food diet.
The problem comes, as has been stated many times, when a cat is on a dry food diet. It can be hard to ensure that your cat maintains an adequate water balance under these circumstances, especially when an older cat’s diet has been changed from wet to dry.
An adequate intake of water is particularly critical when an increase in fluid requirement is required as is the case in a number of diseases such as renal failure. Renal failure is a major illness for elderly domestic cats.
There is no point in measuring out the amount of water that a domestic cat needs to drink daily. The challenge is ensuring that a cat drinks enough water when permanently on dry cat food. The cat owner might need to put down foods containing lots of water such as boiled fish with added water as a counterbalance. Or a water fountain might provide encouragement.
In my experience, from a human perspective, domestic cats are very poor water drinkers. This must be inherited from their wildcat ancestors living in arid conditions in North Africa and Asia.
If an older cat starts to drink a lot of water she may be suffering from kidney disease, an overactive thyroid or diabetes. You should see a veterinarian as soon as possible.