Every day you lose water through your breath, perspiration, urine and bowel movements. For your body to function properly, you must replenish its water supply by consuming beverages and foods that contain water.

So how much water does the average, healthy adult living in a temperate climate need? In general, doctors recommend 8 or 9 cups. Here are the most common ways of calculating that amount:
Replacement approach. The average urine output for adults is about 1.5 liters (6.3 cups) a day. You lose close to an additional liter (about 4 cups) of water a day through breathing, sweating and bowel movements. Food usually accounts for 20 percent of your total fluid intake, so if you consume 2 liters of water or other beverages a day (a little more than 8 cups) along with your normal diet, you will typically replace your lost fluids.
Eight 8-ounce glasses of water a day. Another approach to water intake is the "8 x 8 rule" drink eight 8-ounce glasses of water a day (about 1.9 liters ...more

Many people have heard the recommendation to drink 8 glasses of water per day. However, this guideline was never based on solid scientific evidence (Schnirring L. 2004).

Until recently, about the only definitive research regarding fluid intake and health was a study of Seventh Day Adventists that was published in 2002. Researchers at Loma Linda University in California reported that people who drank at least 5 glasses of water per day had a lower risk of a heart attack during the 6-year period than people who drank 2 or fewer glasses per day (Chan J, et al. 2002).

But even this information is not very precise: how much water is "5 glasses" anyway? To establish better guidelines on fluid intake, a panel of experts from the US and Canada was convened on behalf of the Institute of Medicine. Their report was made public in February 2004