Up to 60% of the body is composed of water. It is used to maintain blood volume, which is required for regulating body temperature and delivering oxygen and nutrients. Water also provides a medium for the biochemical reactions that occur in the cells, and is crucial for the removal of waste products.
Suggested fluid intake is often used interchangeably with suggested water intake, but fluid requirements are met through both foods and drinks. While calorie-free water is the preferred source for fluid, you also consume fluid through fruits, vegetables, and drinks such as coffee, tea, and fruit juices.
There are many methods for calculating daily fluid requirements. One simple equation for adults is a half ounce of fluid per pound of body weight per day. For example, if you weigh 140 pounds, multiply 140 by 0.5 to estimate your daily fluid need in ounces. You can then divide that number by eight to estimate your fluid need in cups per day. [example: 140 x 0.5 = 70 ounces; 70 ounces divided by 8 = 8.75 cups of fluid per day]
0.5 ounces x Body Weight in Pounds = Daily Fluid Requirement in ounces
Another common way to calculate daily fluid need is based on calorie intake -- one milliliter of fluid for every calorie ingested. Converted to ounces, your body needs .034 ounces for every calorie that you ingest.
0.034 ounces x Daily Caloric Intake = Daily Fluid Requirement in ounces
As you calculate your daily fluid requirements, you will likely find that the number is close to the common recommendation of eight to 12 cups per day. The above equations will give you a more accurate guideline because fluid needs vary depending on body size. Also keep in mind that fluid requirements increase under extreme conditions such as exercise in hot and humid weather, and during illness.