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About how much of an adult's body weight is water?
It varies with age, sex and body composition, with higher percentages in infants.
About 60 percent is a rule-of-thumb average for an adult. The USGS gives a range that begins at around 60 percent for adults and notes that individual organs differ widely, with the brain and heart reported at roughly 73 percent water and the lungs near 83 percent, while bones are around 30 percent. Fat tissue contains much less water than lean tissue, which is the main reason the percentage shifts between people.
Because of this, adult men, who on average carry more muscle and less body fat, tend to have a higher water share than women, and the share declines as people age and lose muscle. Newborns are commonly quoted at about 75 percent or more water. Medical references describe body water as split between the fluid inside cells (about two-thirds of the total) and the fluid outside cells, such as blood plasma and the fluid between tissues.
The number should not be confused with how much a person needs to drink, which is a separate and debated topic. Water in the body is constantly turned over through intake, food moisture, metabolism, urine, sweat and breath, so the 60 percent figure describes a stock that is continually replenished, not a fixed supply.