Here is a simple formula you can use. For each pound of body weight, drink 1/2 ounce of water. Note: If you are considerably over- or underweight for your height, you should figure this using a “reasonable” weight for your height. In hot weather or during heavy physical work or exercise, add another ounce of water for every 25 pounds of body weight. For example, if you weigh 150 pounds, try to take in 75 oz (about 9 1/3 cups) a day. When it’s hot or you are doing a lot of exercise, aim for 82 ounces (or about 10 1/4 cups).