Water makes up about 60% of an adult man’s body weight and 52 to 55% of an adult woman’s. That single number is the quick answer, but the full picture is more interesting: water content varies dramatically by organ, shifts across your lifetime, and your body cycles through liters of it every single day.
Why the Percentage Differs Between People
The main reason water percentage varies from person to person is body composition. Fat tissue holds less water than lean tissue, so someone with more muscle carries a higher percentage of water than someone with more body fat. This is why the average for women (52 to 55%) is lower than for men (roughly 60%): women typically carry a higher proportion of body fat.
Age plays a major role too. Newborns are roughly 75% water, which gradually declines through childhood and adulthood. By older age, total body water drops further as people lose muscle mass and gain proportionally more fat. A 70-year-old may be closer to 50% water. So when you hear “the body is 60% water,” that’s a useful average for adult men, not a universal constant.
Water Content by Organ
Not all tissues are created equal when it comes to water. Your lungs are the most water-rich major organ at about 83%. The brain and heart each contain roughly 73% water, and muscles and kidneys sit around 79%. Even bones, which most people picture as dry and rigid, are about 31% water. Healthy bones are nearly one-third water, which helps explain why chronic dehydration can affect bone health over time.
Body fat, by contrast, contains relatively little water. This is the key variable behind all the individual differences: the ratio of lean tissue to fat tissue in your body effectively sets your personal water percentage.
Where All That Water Actually Sits
Your body’s water isn’t just sloshing around freely. About two-thirds of your total body water is inside your cells, forming the fluid environment where all cellular chemistry takes place. The remaining one-third sits outside your cells: in your blood plasma, in the fluid between tissues, and in smaller reservoirs like spinal fluid and the fluid inside your eyes.
This split matters because your body actively manages the balance between these two compartments. Sodium and potassium act as gatekeepers, pulling water into or out of cells to maintain the right concentration on each side. When you’re dehydrated or when electrolyte levels shift, it’s this balance that gets disrupted first.
How Much Water You Cycle Through Daily
Your body doesn’t hold onto the same water indefinitely. A 70-kilogram adult turns over roughly 2,500 to 3,000 milliliters of water per day, which works out to about 4% of total body weight. Half of that loss happens through your lungs and skin without you noticing (breathing out moist air, low-level evaporation through the skin). The other half leaves through urine and, to a small extent, stool.
To replace those losses, you don’t need to drink all of it as plain water. Solid foods contribute about 700 to 800 milliliters daily, roughly 20% of your total intake. Your body also generates around 250 milliliters internally through metabolism, as a byproduct of breaking down food for energy. The rest comes from the fluids you drink.
How Much You Need to Drink
A practical guideline from the University of Michigan Health ties fluid needs to calorie intake: about 1 milliliter of fluid for every calorie you eat. If you eat around 2,000 calories a day, that translates to about 2,000 milliliters, or 8 to 9 cups of fluid. This lines up with the familiar “eight glasses a day” recommendation, though it’s more of a baseline than a rigid target.
Your actual needs shift with activity level, climate, and health. Sweating from exercise or hot weather can easily add a liter or more to your daily requirement. Illness with fever, vomiting, or diarrhea increases losses rapidly. Thirst is a reasonable guide for most healthy adults, though it becomes less reliable with age, since older adults often feel less thirsty even when they need fluids.
The color of your urine is a more practical indicator than counting cups. Pale yellow generally signals adequate hydration. Dark yellow or amber means you’re behind. Clear and colorless for extended periods can mean you’re overhydrating, which in rare cases dilutes blood sodium to dangerous levels.