I have tried many different diets and they all say drink 8 glasses a day or more.  However, it is not necessarily true.

The amount of water you need depends on the weather, the amount of exercise, and your body makeup.  Some people need less water than others.

So why, are diets gun ho about 8 glasses a day?  They are simply trying to change your habits.  They want you reaching for water first before you reach for food.

Source: “Breaking the 8-Glass Myth,” RealAge Diet and Nutrition Center. November 2004.