How much water?

The Replacement Approach is another way of calculating how much water to drink. The average urine output for adults is about 1.5 liters (6.3 cups) a day. You lose an additional liter of water a day through breathing, sweating and bowel movements. Since food accounts for approximately 20 percent of your total fluid intake, you will need to consume 2 liters of water or other beverages a day (a little more than 8 cups) along with your normal diet to replace lost fluids.

Don't start drinking an extra gallon of water a day, especially if you are fasting or eating very little. Water taken in must be in balance with body salt - electrolytes. Though uncommon, it is possible to drink too much water. When your kidneys are unable to excrete the excess water, the electrolyte (mineral) content of the blood is diluted, resulting in low sodium levels in the blood, a condition called hyponatremia. Endurance athletes, such as marathon runners, who drink large amounts of water are at higher risk of hyponatremia. In general, though, drinking too much water is rare in healthy adults who consume an average American diet.