How much fluid you need depends upon several things, including:
Age: Kids need plenty of fluids; they can get dehydrated much more easily than adults. Older people may need more fluids because of health conditions or because they tend to lose their sense of thirst.
Gender: Men need more fluids than women. (And pregnant women need more fluids than other women.)
Weight: Heavier people need more water.
Health: Conditions such as diabetes, cystic fibrosis, and kidney disease can boost your need for fluids.
Environment: You need more fluids in extreme weather conditions (especially hot, humid, or cold) and at high altitudes.
Dehydration occurs when the amount of water in the body is too low. Causes of dehydration include not drinking enough fluids, or losing a lot of fluids through sweating, particularly during hot weather, exercise or a fever, or a combination of these factors.