How much water does a puppy or dog need? A dog needs about ½ ounce to 1ounce per day, so a 20 pound puppy needs about 10 to 20 ounces per day. A 100 pound adult dog needs about 50 ounces a day. Keep in mind there are 128 ounces in a gallon so a big dog needs between a half gallon and gallon or more of water a day to stay hydrated.