Humans with normal health can survive without water for three to five days. The exact amount of time varies based on the size of the person, the environment the person is in and the person's specific state of health.
The human body is 70 percent water. Water is essential to body temperature regulation, proper metabolism and proper organ function. Water is lost continuously through sweat, waste removal and in exhaled air. In hot, dry weather, the human body becomes dehydrated after only a few hours and, without rehydration, may die. Exertion increases this risk. At the other end of the spectrum, a well-hydrated individual with excellent health in a carefully controlled environment can possibly live for six days without water, although his health would be significantly compromised at the end of the period.