The average adult human body is about 50% to 65% water by weight. That means a 150-pound person is carrying roughly 75 to 97 pounds of water at any given moment. The exact percentage varies based on your age, sex, and body composition, but water is by far the most abundant substance in your body.
Why the Range Is So Wide
The 50% to 65% range exists because body fat and lean tissue hold very different amounts of water. Fat tissue contains relatively little water, while muscle and organ tissue are water-rich. This is the main reason men tend to have a higher body water percentage (around 60%) than women (around 52% to 55%), since women on average carry a higher proportion of body fat. A lean, muscular person of either sex will be closer to the 65% end of the range, while someone with more body fat will be closer to 50%.
This also means your body water percentage can shift over your lifetime as your body composition changes, not just because of hydration levels.
How Body Water Changes With Age
You were at your most water-dense as a newborn. Babies in their first month of life are about 75% water by weight. By the time an infant reaches their first birthday, that drops to around 60%. Throughout childhood, body water gradually decreases, reaching adult values around age 12.
At the other end of life, total body water falls again. After age 65, both total body mass and water content decline. This is partly because older adults tend to lose muscle mass and partly because the body’s ability to conserve water becomes less efficient. This reduced water reserve is one reason older adults are more vulnerable to dehydration.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. The lungs, muscles, and kidneys contain the highest percentages, which makes sense given how metabolically active they are. Blood is about 80% water overall, and the plasma portion (the yellowish liquid that carries blood cells) is 90% water. Even your bones, the driest tissue in your body, are still about 31% water. There is no truly “dry” tissue in a living human.
What All That Water Actually Does
Water isn’t just sitting passively in your cells. It plays active roles in nearly every process that keeps you alive. It regulates your body temperature, mainly through sweating and the expansion of blood vessels near the skin. It cushions and protects your organs and tissues, acting as a shock absorber around the brain and spinal cord. And it serves as the primary transport system for nutrients, oxygen, and waste products, carrying them through your bloodstream and helping your kidneys and liver flush out what you don’t need.
At the cellular level, water is the solvent in which virtually all of your body’s chemical reactions take place. Without it, enzymes can’t function, cells can’t maintain their shape, and electrical signals between nerves slow down. Even mild dehydration (losing just 1% to 2% of your body water) can cause noticeable fatigue, difficulty concentrating, and headaches.
How Your Body Maintains Its Water Balance
Your body is constantly losing water through urine, sweat, breathing, and digestion. To stay in balance, the average man needs roughly 15.5 cups of fluid per day, and the average woman about 11.5 cups. That total includes water from food, which typically accounts for about 20% of your daily intake.
Your brain manages this balance with impressive precision. When water levels drop even slightly, a hormone produced in the brain signals the kidneys to hold onto more water, reducing urine output. When you’re well-hydrated, that hormone decreases, and your kidneys release more water as urine. This is why your urine color shifts throughout the day: darker urine generally means the system is conserving water, and pale urine means you have plenty on board.
Thirst itself is a relatively late signal. By the time you feel genuinely thirsty, your body water has already dipped enough to trigger that hormonal response. For most healthy adults, drinking water consistently throughout the day and paying attention to urine color is a more reliable guide than waiting for thirst to kick in.
Factors That Shift Your Percentage
Beyond age, sex, and body composition, several everyday factors influence your body water levels. Exercise increases water loss through sweat and breathing, sometimes by several cups per hour in hot conditions. Altitude and dry climates speed up water loss through your skin and lungs, often without noticeable sweating. Illness involving fever, vomiting, or diarrhea can cause rapid drops in body water.
Certain life stages also matter. Pregnancy increases total body water significantly, both to support the growing fetus and to expand blood volume. People with higher muscle mass, including athletes, consistently test at the higher end of the body water range, sometimes above 60% regardless of sex. On the flip side, gaining body fat without gaining muscle will push your water percentage lower, since each pound of fat holds less water than a pound of muscle.