The human body is roughly 60% water by weight in a typical adult, though that number shifts depending on your age, sex, and body composition. For a 150-pound person, that translates to about 11 gallons of water distributed across every organ, tissue, and cell.
How Body Water Changes With Age and Sex
Newborns carry the highest proportion of water, around 75% of their body weight. That percentage drops steadily through childhood and settles into adult ranges by the teenage years. Adult men average about 60% water, while adult women average closer to 50 to 55%. The gap comes down to body composition: fat tissue holds very little water (only about 5 to 10%), while lean tissue like muscle is water-rich. Because women typically carry a higher percentage of body fat, their overall water proportion is lower.
Body mass index plays a role too. The ratio of total body water to weight decreases as BMI rises, regardless of sex, because extra adipose tissue dilutes the proportion. Older adults also tend to carry less water as a percentage of body weight, partly because lean muscle mass naturally declines with age and body fat percentage increases.
Water Content Across Organs and Tissues
Not all parts of your body hold water equally. The lungs are the most water-dense major organ at about 83%. The brain and heart each sit at roughly 73%, while skeletal muscles and kidneys come in at about 79%. Even your bones contain a meaningful amount of water. At the far end of the spectrum, tooth enamel is only about 3% water by weight, making it the driest tissue in the body.
This distribution matters because it helps explain why dehydration affects some functions faster than others. Your brain, being nearly three-quarters water, is particularly sensitive to fluid loss, which is why headaches, difficulty concentrating, and fatigue are among the earliest signs that you need more fluids.
Where That Water Actually Sits
Your body stores water in two main compartments. About 60% of your total body water is intracellular, meaning it sits inside your cells, where it serves as the medium for virtually every chemical reaction keeping you alive. The remaining third is extracellular: circulating in your blood plasma, filling the spaces between cells, and cushioning your spinal cord and joints.
Your body works constantly to keep the balance between these two compartments stable. When you drink fluids, water is absorbed through your digestive tract into the bloodstream and then distributed where it’s needed. Your kidneys fine-tune the balance by adjusting how much water you retain or excrete, responding to hormonal signals that track your hydration status in real time.
What All That Water Does
Water isn’t just filling space. It performs a long list of active jobs:
- Temperature regulation. When you overheat, your body pushes water to the skin’s surface as sweat. As it evaporates, it pulls heat away from your body.
- Nutrient and oxygen transport. Blood plasma, which is mostly water, carries dissolved nutrients, oxygen, and hormones to cells throughout the body.
- Waste removal. Water helps your kidneys and liver flush metabolic waste products out through urine.
- Joint lubrication. The fluid in and around your joints reduces friction and absorbs shock during movement.
- Tissue protection. Water moistens delicate tissues in your eyes, nose, and mouth, and cushions organs against physical impact.
- Digestion. Saliva and digestive fluids are water-based, and water helps dissolve minerals and nutrients so your body can absorb them.
How Much You Need to Replace Each Day
You lose water continuously through breathing, sweating, urination, and digestion. To maintain that 60% balance, the average healthy adult needs roughly 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. That number includes water from all sources: plain water, other beverages, and food. Fruits, vegetables, soups, and other water-rich foods can contribute 20% or more of your daily intake.
Your actual needs shift with activity level, climate, illness, and pregnancy or breastfeeding. Heavy exercise in hot weather can easily double the baseline requirement. Thirst is a reasonable guide for most healthy adults, though it tends to lag slightly behind actual need, especially in older adults whose thirst signals weaken with age.
How Body Water Is Measured
If you’ve ever wondered how scientists pin down these numbers, the gold standard involves drinking a small, harmless dose of labeled water (a form of water with a trackable hydrogen marker) and then measuring how it dilutes across the body over about two hours. By comparing the concentration in saliva or blood plasma to the known dose, researchers can calculate total body water with high precision. Studies using this method consistently find that total body water represents about 74% of fat-free mass, which is why body composition is the single biggest factor in individual variation.
A simpler, more common method used in clinics and gyms is bioelectrical impedance, where a weak electrical current is passed through the body. Water conducts electricity well, so the device estimates total body water based on how easily the current travels. It’s less precise than the tracer method but practical enough for routine monitoring.