In physiology, body water is the water content of the human body. A significant fraction of the human body is water.
Arthur Guyton 's Textbook of Medical Physiology states that "the total amount of water in a man of average weight (70 kilograms) is approximately 40 litres, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 75 percent of the body weight, but it progressively decreases from birth to old age, most of the decrease occurring during the first 10 years of life. Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent". These figures are statistical averages, so are illustrative, and like all biostatistics, will vary with things like type of population, age and number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure. For example, Jackson's (1985) Anatomy & Physiology for Nurses gives a figure of 60% for the proportion of body-weight attributable to water, which approximates Guyton's 57%.