Mayo Clinic defines adequate intake of fluids as approximately 13 cups, or 104 ounces for an adult male and 9 cups, or 72 ounces for an adult female. Many other factors can change this number, but as a starting point these numbers provide a good guide.
The nature of the fluid being consumed, its general make-up and what other substances are suspended in the liquid can change how much water is actually contributing to the body's function, according to Mayo Clinic. High salinity water or fluids with diuretic components, such as caffeine or sugar, will result in mild dehydration due to their effects on the body. Likewise, a person's activity level also affects how much water is necessary to maintain healthy bodily functions. Highly active individuals are advised to consume more water in order to replenish those fluids lost as perspiration. Further, illness or health in general can play a role in a person's water needs. People suffering from gastrointestinal illness which causes diarrhea, vomiting or sweating require more water due to the rapid fluid loss caused by these symptoms. Finally, age plays a part in determining how much water is adequate. For example, children simply have lower total body mass, so they need less total water volume in order to properly hydrate.