Water makes up about 50% to 65% of your total body weight. For an average 154-pound (70 kg) adult, that translates to roughly 77 to 100 pounds of water, or about 9 to 12 gallons. The exact amount varies depending on your age, sex, and body composition.
Why the Percentage Varies From Person to Person
Fat tissue holds less water than lean tissue. Because adult men typically carry more muscle mass, their bodies tend to sit closer to 60% to 65% water, while adult women average around 50% to 55%. This is also why two people who weigh the same can have noticeably different total body water levels if one carries more body fat.
Age plays a major role, too. Newborns are roughly 75% water. That percentage gradually declines through childhood and adolescence, then continues to drop in older adulthood as muscle mass decreases and body fat tends to increase. A person in their 70s or 80s may fall below 50%.
How Water Is Distributed Across Your Organs
Water isn’t evenly spread throughout your body. Some organs are surprisingly water-dense, while others you might expect to be dry still contain a significant amount. Data from the U.S. Geological Survey breaks it down:
- Lungs: 83% water
- Muscles and kidneys: 79%
- Brain and heart: 73%
- Skin: 64%
- Bones: 31%
Blood, which is roughly 90% water by volume in its plasma component, acts as the transport network. It carries dissolved nutrients, oxygen, and waste products between every tissue on that list. Even your bones, the driest major tissue, rely on that 31% water content for flexibility and mineral exchange.
What All That Water Actually Does
Your body uses water for far more than just quenching thirst. It’s involved in virtually every chemical reaction that keeps you alive, and it plays several structural roles that are easy to overlook.
Temperature Control
Your body maintains a core temperature of about 98.6°F (37°C), and water is central to keeping it there. When you overheat, blood vessels near the skin dilate and sweat glands release water to the surface, where evaporation pulls heat away. Water’s high heat capacity means it can absorb a lot of thermal energy before its temperature changes, which buffers your internal organs from rapid temperature swings. If core temperature drops to around 82°F (28°C), muscles begin to fail and hypothermia sets in, so this buffering effect is not a minor convenience.
Cushioning and Lubrication
The fluid between your joints is primarily water, allowing bones to glide against each other without grinding. Cerebrospinal fluid, which is mostly water, surrounds your brain and spinal cord, cushioning them against impact. The fluids inside your eyes serve the same protective function. In pregnancy, the amniotic sac fills with water-based fluid just two weeks after fertilization, providing a shock absorber for the developing embryo.
Immune Defense and Tissue Protection
Mucus, which is over 90% water, lines your respiratory and digestive tracts. It traps pathogens and irritants before they reach vulnerable tissue and contains immune cells that destroy invaders on contact. Watery fluids surrounding your internal organs provide both chemical and mechanical protection, essentially acting as a built-in padding system.
How Much Water You Lose Each Day
Your body cycles through a surprising volume of water every 24 hours. According to data compiled by Harvard Medical School’s BioNumbers database, the average adult loses about 2,550 milliliters (roughly 86 ounces) of water per day through four main routes:
- Urine: 1,500 ml (about 51 oz), the largest single source of water loss
- Insensible loss through skin and lungs: 900 ml (about 30 oz), water that evaporates from your skin and exits with every breath you exhale, even when you’re not sweating
- Feces: 100 ml
- Sweat: 50 ml at rest, though this number climbs dramatically during exercise or in hot environments
That 50 ml sweat figure represents a sedentary person in a comfortable climate. During intense exercise, sweat losses alone can reach 1 to 2 liters per hour, which is why athletes need to pay closer attention to fluid replacement than someone sitting at a desk.
Keeping the Balance
Your body is remarkably good at signaling when water levels drop. Thirst kicks in when you’ve lost as little as 1% to 2% of your body water, and your kidneys constantly adjust how much water they retain or release in urine. When you’re well hydrated, urine is pale and dilute. When you’re running low, the kidneys concentrate it to conserve fluid, which is why dark yellow urine is a reliable early sign of dehydration.
You take in water from three sources: beverages, food, and a small amount generated internally by your metabolism. Fruits, vegetables, soups, and even foods like bread contribute meaningful water. For most people, about 20% of daily water intake comes from food rather than drinks. The remaining 80% comes from whatever you sip throughout the day.
Because daily losses average around 2.5 liters, your intake needs to at least match that to stay in balance. Physical activity, heat, illness, and altitude all push losses higher, so your needs shift day to day. Pale straw-colored urine and the absence of thirst are the two simplest indicators that you’re keeping up.