The latest dietary reference intake report by the United States National Research Council in general recommended (including food sources): 2.7 liters of water total for women and 3.7 liters for men.
Reference daily intake for water is 3.7 L/day for human males aged 19-30 years. Food contributes 0.5–1 L, and the metabolism of protein, fat, and carbohydrates produces another 0.25–0.4 L. Thus, a person needs to drink approximately 2–3 L of water per day.
Dehydration is best avoided by drinking plenty of water. The greater the amount of water lost through perspiration, the more water must be consumed to replace it and avoid dehydration. Since the body cannot tolerate large deficits or excesses in total body water, consumption of water must be roughly concurrent with the loss (in other words, if one is perspiring, one should also be drinking water frequently). Drinking water beyond the needs of the body entails little risk, since the kidneys will efficiently remove any excess water through the urine with a large margin of safety.
Drink at least half a liter of water before exercise to ensure your fluid levels are up to start and then continue taking in as much as you can during exercise. Also, always try to drink more fluids than you need. Smaller quantities at frequent intervals helps optimize hydration.
Water is the most abundant, yet possibly the most overlooked, substance in the body, as it represents 45 to 60% of an adult's total body weight. It plays a vital role in all bodily processes by providing a universal medium for chemical reactions, lubrication, nutrient delivery, waste disposal, heat dispersion and temperature regulation.
In order to maintain the fluid balance within the body water gain needs to equal water loss. Exercise significantly increases fluid loss which can reach levels of up to 4 liters per hour during heavy work-outs. A loss of body mass through dehydration of as little as 2% significantly impairs performance which emphasizes the importance of the maintenance of hydration, especially during exercise.