I’d like to take a look at this question in a couple of different ways. First, I used a well-regarded “calculator” to determine how much water a 10 pound, adult neutered dog and cat should take in per day. The results were:

Dog: 348 +/- 70 mls/day

Cat: 261 +/- 52 mls/day

These results support the idea that in comparison to dogs, cats simply need relatively less water per pound of body weight.

Now let’s look at where our 10 pound kitty can get that water. This cat needs approximately 261 kcal of energy from food per day. (That’s not a typo. A general rule of thumb states that water needs in ml are the same as caloric needs in kcal.) I am going to use a major pet food manufacturer’s adult maintenance feline foods (both dry and canned) as representative diets for our water calculations.

The dry food contains 502 kcal/cup. Therefore, our kitty should be eating 0.52 cups per day. Dry food typically contains about 10% water so 0.52 cups of food would provide 0.052 cups of water or 12.3 mls. Subtracting that from our 261 mls per day leaves us with 249 mls (or about one cup) of water that the cat needs to drink from a bowl per day.

The company’s canned food contains 88 kcal/85 g can, so our kitty should be eating about three cans (these are tiny cans!) per day (88 x 3 = 264 kcal). Most canned cat food contain between 68 and 78 percent water. I’ll use the average of 73% here. So, 73% of 85 grams times 3 is 186 grams of water, which equals 186 mls of water. Subtracting 186 mls from the cat’s 261mls total daily water need leaves 75 mls (or roughly one third of a cup).

Is your brain spinning from all that math? Sorry! The take home message is that when cats eat dry food, they need to get almost all of their water from sources other than their food, while a canned food only diet can supply around two-thirds of a cat’s needs.

This difference could be critical if cats truly do have a low thirst drive … more on this (and less math!) next week.

Dr. Jennifer Coates