The human body is roughly 50 to 60 percent water by weight. That means more than half of you, right now, is water. For an average adult male, that works out to about 40 liters (over 10 gallons) sloshing around inside, playing roles in everything from temperature control to thinking clearly.
The exact percentage depends on your age, sex, and body composition. Adult men average around 60 percent water, while adult women average closer to 50 percent, largely because women tend to carry more body fat, which holds less water than muscle. Newborns are the most water-dense humans at roughly 75 percent, while older adults gradually decline as muscle mass decreases with age.
Where All That Water Actually Sits
Your body’s water isn’t just floating around freely. About two-thirds of it, roughly 25 liters in an average adult, is locked inside your cells. This intracellular fluid is where most of your body’s chemistry happens: energy production, protein building, waste processing. The remaining third, about 15 liters, exists outside your cells. This extracellular fluid includes blood plasma, the liquid cushion between cells, and the fluid surrounding your brain and spinal cord.
That split matters because your body constantly works to keep the balance right. When you’re dehydrated, water shifts between these compartments to protect critical functions, which is why mild dehydration can cause symptoms long before you’re in any medical danger.
Some Organs Hold Far More Water Than Others
Not every part of your body is equally watery. Your lungs are the most water-rich major organ at 83 percent water, which makes sense given that they need to stay moist to exchange oxygen and carbon dioxide with every breath. Your brain and heart are both about 73 percent water. Muscles and kidneys run close to 79 percent, and your liver sits at 71 percent.
Blood is 90 percent water, and both saliva and sweat are essentially water at 99 percent. At the other end of the spectrum, bones are only about 31 percent water. Even your skeleton, as solid as it feels, contains a meaningful amount of fluid in the living tissue within it.
What All That Water Does for You
Water isn’t just filling space. It’s actively doing work in nearly every system of your body. It carries nutrients and oxygen to your cells through the bloodstream. It cushions and lubricates your joints, which is one reason dehydration can make your joints feel stiff. It keeps the tissues in your eyes, nose, and mouth moist. It helps your kidneys and liver flush waste products out of your blood, and it dissolves minerals and other nutrients so your body can actually absorb and use them.
One of water’s most critical jobs is temperature regulation. When you overheat, your body pushes water to the skin’s surface as sweat. As that sweat evaporates, it pulls heat away from your body. This cooling system is remarkably effective, but it depends on having enough water to work with. It’s also why you need to drink significantly more in hot weather or during exercise.
How Much Water You Lose Every Day
Your body is constantly cycling through its water supply. On a normal day without heavy exercise, you lose close to a liter of water just through evaporation from your skin and the moisture in every breath you exhale. Your kidneys account for the biggest share of water loss, producing anywhere from about half a liter to over 10 liters of urine daily depending on how much you drink and what your body needs to flush out.
Vigorous exercise, hot weather, or a fever can dramatically increase water loss through sweating. Vomiting or severe diarrhea can cost you several liters in a single day. To keep everything in balance, healthy adults generally need at least about 2 liters (roughly half a gallon) of fluids daily, though the actual amount varies with activity level, climate, and body size.
What Happens When You Lose Even a Little
You don’t need to be severely dehydrated to feel the effects. Research published in The Journal of Nutrition found that losing just 1.36 percent of body water in young women was enough to degrade mood, increase the perception that tasks were difficult, reduce concentration, and trigger headaches. These effects showed up both at rest and during exercise.
That 1.36 percent loss, for someone with 30 liters of total body water, is less than half a liter. Roughly one tall glass of water. By the time you feel thirsty, you’ve typically already crossed into mild dehydration, which is why steady fluid intake throughout the day tends to work better than waiting until thirst hits.
The fact that such a small shift can affect your thinking and mood reflects just how precisely your body manages its water balance. You are, in the most literal sense, a carefully regulated container of water, and keeping that container topped off is one of the simplest things you can do for your physical and mental performance.