“Water needs in ml are the same as caloric needs in kcal”
And a “kcal” is a regular, dietary Calorie that we’re all familiar with.
That means if your cat eats 300 Calories of food per day, it should get 300 mL (1.3 cups) of water per day (including all sources of water, including food). Simple!
Method 2: based on cat weight
According to VetWest, cats should drink 60 mL of water per kg of cat weight (27.3 mL per pound weight). So you can simply weigh your cat, and multiply.
How many calories is my cat eating?
So to calculate water requirements, you need to estimate the amount of calories your cat gets daily.
Method 1: if your cat eats all its food
If your cat always eats everything in its bowl, your job is easy: Just keep track of the amount you feed it. If you feed it three cans of wet food a day, check the can to see how many calories are in each can. If you feed it dry food, weigh the amount you give it, and again see the label to learn how many Calories that provides. Do this over a few days to get an accurate estimate.
Method 2: if your cat just eats all day
If you just keep the cat bowl filled with dry food, it’s also easy: fill the bowl in the morning, and weigh the bowl plus food together. Then weigh the bowl 24 hours later, and subtract the weights. Check the food label to see how many Calories were in the amount consumed. Do this over a few days to get an accurate estimate.
Method 3: estimate by cat weight
If your cat gets food from several sources, you have several cats sharing the same food, or it’s too difficult to weigh, use the below table (numbers shown are Calories):
Daily Calorie Requirements for Cats. Source: https://www.animalmedicalcenterofchicago.com/pdf/CalorieRequirementsForCats.pdf
How to calculate cat water intake from Calorie consumption
Once you know the amount of Calories your cat consumes daily, use one of the below formulas to calculate daily water requirements:
Water (mL) = Calories
Water (US fluid ounce) = Calories/29.5
Water (US customary cups) = Calories/237
How much water is my cat drinking?
Now that you know how much your cat should drink, you can consider how much it is drinking.
Measure the water bowl
If you have one cat that drinks from a water bowl, just measure how much water you add to fill the bowl. After 24 hours, empty the remaining water into a measuring cup, and subtract the difference.
Consider if your cats drink from other sources (such as a toilet or sink) and restrict these sources while you measure.
If you have several cats that share the same water bowl, you may need to separate them while measuring, in a way appropriate for your situation.
Try not to change anything while testing, such as using a different water bowl, which may change your cat’s drinking behavior.
Don’t forget about water from food sources: see below!
Water from cat food sources
Food is the other major source of water for your cat, so remember to include it in your estimate of total water intake.
As remarkable as it may seem, the domestic cats that currently share our homes and fill our hearts evolved from a desert-dwelling species. Though there are many differences between those original ancestors and the cats we currently keep as our pets, one thing that has not changed is the ability of our domestic cats to produce strongly concentrated urine as well as a low thirst drive in many cats.
Cats that do not stay hydrated may suffer from urinary tract disease, including kidney disease and lower urinary tract disease, which can be present in many forms. Inflammation of the bladder (cystitis) is common. Bladder stones are possible and can lead to life-threatening urethral blockages, particularly in male cats. Contrary to popular belief, urinary tract infection is actually not very typical in cats less than 10 years of age. However, younger cats can suffer from various other urinary tract diseases.
According to Lisa A. Pierson, DVM, a California veterinarian focused on feline medicine and nutrition, the biggest (nutritional) mistake people make is feeding cats dry food:
“We know that a cat’s sensitivity to thirst is blunted compared to a dog,” Case says. “They don’t voluntarily drink water like a dog would.” And because cats naturally produce highly concentrated urine “we’re setting them up for urinary tract problems when their diet is low in liquids.”