About 60% of the adult human body is water. For a 70 kg (154-pound) person, that works out to roughly 42 liters, or about 11 gallons. But that number shifts depending on your age, sex, and body composition, and it plays a role in nearly every process that keeps you alive.
Water Percentage by Age and Sex
Children carry the most water relative to their size. Between ages 3 and 10, both boys and girls are about 62% water. The numbers start diverging at puberty, when girls develop more body fat (which holds less water than muscle). By the teen years, females drop to around 55-56% while males stay near 62-63%.
That gap holds through most of adulthood. Normal-weight men typically remain around 62% water from their twenties through their fifties, while normal-weight women hover around 54%. After age 60, both sexes see a decline. Men drop to about 57%, and women to around 50%. This happens partly because older adults tend to lose muscle mass and partly because the body’s ability to regulate fluid balance weakens with age.
Why Body Composition Matters
The single biggest factor driving individual differences in body water percentage is the ratio of muscle to fat. Muscle is about 76% water. Fat tissue holds considerably less. This is why two people of the same weight can have very different total body water levels: a muscular person carries more water than someone with a higher body fat percentage.
It also explains the sex difference. Women naturally carry more essential body fat than men, so their overall water percentage runs lower. Athletes of either sex tend to land on the higher end of the range because of their greater muscle mass.
Where the Water Actually Sits
Your body water isn’t evenly distributed. About two-thirds of it (roughly 28 liters in an average man) sits inside your cells, forming what’s called intracellular fluid. This is the water that fills every cell in your body, from neurons to muscle fibers, and it’s essential for the chemical reactions that keep those cells working.
The remaining third (about 14 liters) is extracellular fluid. That includes blood plasma, the fluid between your cells, and smaller volumes in places like the spinal cord and eyes. Your kidneys constantly fine-tune the balance between these two compartments by adjusting how much water you retain or excrete.
Water Content of Individual Organs
Some organs are far more water-dense than you’d expect. The lungs top the list at about 83% water, which makes sense given that they need to stay moist for gas exchange. The brain and heart are both around 73%. Muscles and kidneys come in at 79%, and skin at 64%. Even bones, which feel completely solid, are roughly 31% water.
This is one reason dehydration can affect so many different body systems at once. When total body water drops, every organ with a high water content is affected.
What All That Water Does
Water serves as the medium for virtually every biological process in the body. Its roles fall into a few major categories.
Temperature control. When you overheat, your body pushes water to the skin’s surface as sweat. As the sweat evaporates, it cools you down. In sedentary conditions, you lose about 0.3 liters per hour through your skin. During intense exercise in the heat, that can spike to 2 liters per hour. Without enough incoming fluid to replace those losses, your core temperature rises.
Waste removal. Your kidneys filter waste products from the bloodstream and flush them out through urine. This process requires a minimum volume of water, and the kidneys can produce up to about 1 liter of urine per hour at maximum output. When you’re dehydrated, the kidneys conserve water by concentrating the urine, but there’s a limit to how much they can compensate.
Blood pressure and circulation. Blood volume depends directly on hydration. Drinking water can measurably raise blood pressure and lower heart rate within 15 to 20 minutes, with effects lasting up to an hour. This is why significant dehydration can cause lightheadedness and a rapid pulse: there’s simply less fluid in the system for the heart to pump.
Water also cushions the brain and spinal cord, lubricates joints, and acts as the solvent that carries nutrients into cells and carries waste out.
How Quickly Dehydration Affects You
You don’t need to lose much water before things start going wrong. Your thirst sensation kicks in at just 1-2% body water loss. At that same range, both physical and mental performance can already start to decline. For a 150-pound person, 1-2% loss translates to losing roughly 0.7 to 1.4 liters of water, which is easy to hit during a long workout or a hot day without enough fluids.
At 2% loss and above, cognitive effects become more reliable and pronounced: slower reaction times, difficulty concentrating, and impaired short-term memory. Physical endurance drops too. The effects compound from there, and losses above 2% increasingly strain the cardiovascular system and thermoregulation.
How Much You Need to Replace
Under normal conditions, adults need roughly 2.5 to 3 liters of total fluid per day. Current guidelines suggest about 11.5 cups (2.7 liters) for women and 15.5 cups (3.7 liters) for men from all sources combined, including food. Fruits, vegetables, soups, and other water-rich foods typically account for about 20% of your daily intake.
Those numbers can climb dramatically with heat and exercise. In extreme conditions, daily water needs can reach 6 liters. The key variable is sweat rate, which depends on temperature, humidity, how hard you’re working, and even what you’re wearing. If you’re active in hot weather, thirst alone may not keep up with your actual losses, making it worth tracking your intake more deliberately.