For years we've been told that drinking 8 cups a day is necessary. However, it seems that this advice is incorrectly based on a recommendation of the National Academy of Science that was published in 1940. The publication claimed that a person must drink one millimeter of for every calorie you burn a day - a little more than 8 cups (2000 calories). That same recommendation said that fluids in food count for some of these 8 cups.
In a survey that was conducted in 2000 for Rockefeller University and the International Bottled Water Association found that most adults in the 14 cities polled drank 6 cups of water a day. They found that together with coffee, tea and other drinks, these same adults actually drink 17.6 cups of fluid a day.
A doctor from Dartmouth Medical Center found that not only is the recommendation of 8 cups of water a day not scientifically supported, but that there is no need for a person to drink that much water a day.