Water makes up about 55% to 60% of the average adult human body. That number shifts depending on your age, sex, and body composition, but for most adults it lands in that range. An adult man averages around 60%, while an adult woman averages closer to 55%.
How Water Percentage Changes With Age
You start life as mostly water. Newborns are roughly 75% to 78% water, which drops to about 65% by age one as body composition shifts. Through childhood and into adulthood, the percentage continues to decline gradually, settling into the 55% to 60% range for most healthy adults.
In older adults, total body water drops further. The main reason is straightforward: aging reduces muscle mass, and muscle is one of the body’s primary water reservoirs. With less muscle to store water, your overall percentage falls. This is also why older adults become dehydrated more easily, since there’s simply less of a buffer to draw from.
Why Men and Women Differ
The gap between men (about 60%) and women (about 55%) comes down to body fat. Fat tissue contains only about 14% water, while fat-free mass (muscle, organs, bone) is roughly 80% water. That’s nearly a sixfold difference in water content per unit of weight. Because women on average carry a higher percentage of body fat than men, their overall water percentage is lower. For the same reason, two people of the same sex and age can have noticeably different body water percentages if one is leaner and more muscular than the other.
Water Content of Individual Organs
The 55% to 60% figure is a whole-body average, but individual organs vary widely. According to the U.S. Geological Survey, the lungs top the list at about 83% water. Muscles and kidneys come in at 79%, while the brain and heart are each about 73%. Skin sits at 64%. Even bones, which feel completely solid, are 31% water.
These numbers help explain why dehydration affects so many systems at once. When your body’s water supply drops, organs that depend heavily on water for normal function, particularly the brain, kidneys, and muscles, are among the first to show the strain.
Where All That Water Lives
Your body doesn’t store water in one big tank. Roughly two-thirds of it sits inside your cells, in what’s called intracellular fluid. The remaining third is extracellular, meaning it’s outside the cells. Of that extracellular portion, about 75% fills the spaces between cells in your tissues, and the other 25% is the plasma in your bloodstream. In practical terms, that plasma portion accounts for about 5% of your total body mass.
This distribution matters because your body constantly shuttles water between compartments to maintain blood pressure, deliver nutrients, and keep cells functioning. When you’re dehydrated, the body pulls water from less critical areas to protect blood volume and organ function.
What Water Actually Does in Your Body
Water isn’t just filling space. It serves as the solvent for nearly every chemical reaction in your cells, carrying nutrients to tissues and waste products away from them. It lubricates your joints and forms the basis of saliva. It regulates your temperature through sweating: as sweat evaporates from your skin, it pulls heat away from the body, which is the primary cooling mechanism during exercise or hot weather.
Water also cushions sensitive structures. The fluid surrounding your brain and spinal cord acts as a shock absorber. Amniotic fluid protects a developing fetus in the same way. Even the thin film of fluid lining your lungs helps them expand and contract smoothly with each breath.
How Much Water You Need Daily
General guidelines suggest healthy adults need about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, with the higher end applying to men. “Total fluid” includes water from food, which typically accounts for about 20% of your daily intake. Fruits, vegetables, soups, and even coffee all contribute.
Your actual needs shift with activity level, climate, illness, and pregnancy or breastfeeding. Thirst is a reasonable guide for most healthy adults, though it becomes less reliable with age, which is one more reason older adults are at higher risk for dehydration.
How Body Water Is Measured
If you’ve ever stepped on a smart scale that estimates body water, it likely uses a method called bioelectrical impedance analysis, or BIA. The device sends a tiny electrical current through your body. Water conducts electricity well, while fat does not, so the device estimates your total body water based on how easily the current passes through.
BIA is convenient and noninvasive, but it has limits. In clinical validation studies, BIA predicted total body water with a margin of error of roughly 2 to 3 liters compared to the gold standard (a technique using isotope-labeled water). That’s accurate enough to track trends over time, but variables like how hydrated you are at the moment of measurement, whether you’ve just exercised, or even whether you’re standing or lying down can skew a single reading. The gold-standard isotope dilution method is far more precise but only used in research settings.
What Dehydration Looks Like by Severity
Dehydration is classified by how much body weight you’ve lost through fluid. In infants, mild dehydration is up to 5% of body weight lost, moderate is 6% to 10%, and severe is 10% to 15%. In older children and adults, the thresholds are lower: mild is up to about 3%, moderate around 6%, and severe around 9%.
Even mild dehydration, losing just 1% to 2% of your body weight in water, can cause noticeable effects: headaches, difficulty concentrating, fatigue, and darker urine. By the time you feel genuinely thirsty, you may already be mildly dehydrated. Moderate to severe dehydration brings dizziness, rapid heartbeat, confusion, and in extreme cases, organ damage. For most people, staying ahead of thirst and paying attention to urine color (pale yellow is the target) is enough to stay well hydrated.