Adults need to drink at least 1.5 liters of fluid a day!
Or more if it's very hot or they are physically active. Water is the best source, of course, and we can use tap or mineral water, sparkling or non-sparkling, plain or flavored. Fruit juices, tea, soft drinks, milk, and other drinks, can all be okay - from time to time.
Lack of an appropriate quantity of water may cause many disorders, so it is recommended to take a proper quantity of water daily.
The general rule is to drink half your body weight in ounces per day.