Water in the human body

Roughly 55 to 60% of an adult body is water, distributed between cells, the space around them, and blood plasma — and the proportions vary a lot between people.

Why the range: Muscle is roughly 75% water; adipose tissue is roughly 10%. Body composition therefore matters more than body size.
By age: Newborns are around 75% water. The proportion falls through childhood and continues to decline in later life.
Balance, not stock: The body regulates concentration, not volume, principally through the kidneys and antidiuretic hormone. Thirst is a reliable signal for most healthy adults under normal conditions.

These are population averages. Individual values vary substantially, and clinical assessment of hydration status uses measurements rather than these proportions.

Text description

Total body water is about 55 to 60% of adult body mass. About two thirds of it is intracellular, inside cells. The remaining third is extracellular, of which most is interstitial fluid between cells and roughly a quarter is blood plasma. The proportion is higher in infants, lower in older adults, and lower in people with a higher proportion of body fat, because adipose tissue holds much less water than muscle.