The average adult human body is about 55% to 60% water by weight. For a 154-pound (70 kg) person, that translates to roughly 38 to 42 liters of water, or about 10 to 11 gallons. The exact percentage depends on your age, sex, and body composition, with some people carrying noticeably more or less water than others.
Why the Number Varies From Person to Person
Body fat is the single biggest reason water percentages differ between individuals. Lean tissue (muscle, organs, connective tissue) is about 70% water by weight, while fat tissue holds only about 14 to 20% water. That means a muscular person carries proportionally more water than someone with a higher body fat percentage, even if they weigh exactly the same.
This also explains much of the sex-based difference. Adult men typically fall in the 57% to 60% range, while adult women tend to be closer to 49% to 55%, largely because women on average carry a higher proportion of body fat. During puberty, this gap widens. Boys maintain a water-to-weight ratio around 55% to 59% through their teenage years, while girls see theirs decline from about 53% to 49% by age 16 as body composition shifts.
How Water Content Changes With Age
Newborns are the most water-rich humans, at roughly 75% water by body weight. That percentage drops steadily during the first year of life and continues declining gradually into adulthood. Older adults tend to carry less total body water than younger adults, partly because muscle mass decreases with age and is replaced by tissue that holds less water. A person in their 70s or 80s may be closer to 50% water or even lower, which is one reason older adults are more vulnerable to dehydration.
Where All That Water Sits
Your body water isn’t sloshing around freely. It’s distributed across two main compartments in roughly a 2:1 ratio. About two-thirds sits inside your cells, forming the medium where most of your body’s chemical reactions take place. The remaining one-third is outside cells.
That extracellular water splits further. About 75% of it (roughly 15% of your total body mass) is interstitial fluid, the liquid that surrounds cells in tissues throughout your body. The remaining 25% (about 5% of body mass) is plasma, the liquid portion of your blood. Together, these compartments create a tightly regulated system that your body works constantly to keep in balance.
What Water Actually Does in Your Body
Water isn’t just filling space. It’s the transport system for nutrients and waste. The carbohydrates and proteins you eat are broken down and carried to cells through the bloodstream, which is mostly water. Carbon dioxide and other metabolic waste travel the same route back out.
Water also acts as your body’s cooling system. When your core temperature rises, blood vessels near the skin dilate and sweat glands release water to the surface, where evaporation pulls heat away. Even the moisture in each exhaled breath carries a small amount of heat with it. Beyond temperature control, water cushions your brain and spinal cord, lubricates your joints, and serves as the solvent for nearly every biochemical reaction keeping you alive.
How Much Water You Lose Daily
Your body sheds water continuously through four main routes. The kidneys produce 1 to 2 liters of urine per day. Your skin loses about 450 milliliters through insensible perspiration (the moisture that evaporates before you ever notice you’re sweating) in a temperate climate, and far more during exercise or heat. Breathing accounts for another 250 to 350 milliliters daily, and your digestive tract releases about 200 milliliters through stool. Add it all up and you’re losing roughly 2 to 3 liters a day under normal conditions, more if you’re active, in a hot environment, or at altitude.
To replace those losses, most healthy adults need about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid daily from all sources combined. That includes water in food, which typically accounts for about 20% of your intake. Fruits, vegetables, soups, and even meats contribute meaningful amounts of water.
When Water Balance Tips
Dehydration is measured as the percentage of body weight lost through fluid. In children, losing just 3% of body weight through water loss counts as mild dehydration, 6% is moderate, and 9% is severe. Infants tolerate slightly more before symptoms worsen (up to 5% for mild, 6 to 10% for moderate). In adults, a 1% to 2% loss is enough to impair concentration, increase fatigue, and trigger thirst. By the time you feel genuinely thirsty, you’re typically already mildly dehydrated.
The early signs are subtle: darker urine, slight headache, dry mouth, reduced energy. More significant dehydration brings dizziness, rapid heartbeat, and confusion. Because older adults have less total body water to start with and often have a blunted thirst response, they can progress from mild to serious dehydration faster than younger people.
How Body Water Is Measured
If you’ve ever stepped on a smart scale that reports your body water percentage, it used a technique called bioelectrical impedance analysis, or BIA. The device sends a tiny electrical current through your body and measures resistance. Water conducts electricity well, so tissues with more water (like muscle) allow current to pass more easily than tissues with less water (like fat). From those resistance readings, the device estimates your total body water.
BIA is portable, inexpensive, and takes about 90 seconds. A National Institutes of Health panel concluded it provides a reliable estimate of total body water under most conditions. That said, readings can shift based on factors that have nothing to do with your actual hydration: skin temperature, recent exercise, a full bladder, recent meals, even the temperature of the room. For the most consistent results, measure at the same time of day under similar conditions.
The gold standard for researchers is isotope dilution, where a person drinks water containing a traceable form of hydrogen or oxygen. Scientists then measure how much that tracer gets diluted across the body to calculate total water volume precisely. It’s accurate but expensive, time-consuming, and impractical outside a research setting.