The average adult human body is about 55 to 60% water by weight. For a 70-kilogram (154-pound) person, that translates to roughly 38 to 42 liters of water, enough to fill a large bathtub halfway. The exact percentage depends on your age, sex, and how much body fat you carry.
How Sex and Body Fat Shift the Number
Men average around 60% water by body weight, while women typically fall between 52 and 55%. The difference comes down to body composition. Fat tissue holds only about 14% water, while lean tissue (muscle, organs, connective tissue) is roughly 70% water. Because women tend to carry a higher proportion of body fat, their overall water percentage is lower. Two people who weigh the same can have very different total body water if one is more muscular and the other carries more fat. A lean, athletic person of any sex will be closer to the higher end of the range.
How Water Content Changes With Age
Newborns are the most water-dense humans, at about 75% water by weight. By the time a baby reaches one year old, that drops to around 60%. From there, body water gradually settles into adult ranges by about age 12.
At the other end of life, total body water falls again. After age 65, both total body mass and water content decline. Older adults lose muscle mass over time, and since muscle is one of the most water-rich tissues in the body, total water percentage drops with it. This 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. Some organs are surprisingly wet, while others are relatively dry.
- Lungs: 83% water, the most water-rich major organ
- Brain and heart: 73% water each
- Muscles and kidneys: 79% water
- Skin: 64% water
- Bones: 31% water, even your skeleton carries a meaningful amount
The high water content of the brain and heart helps explain why even mild dehydration can affect concentration, mood, and cardiovascular function relatively quickly.
Where All That Water Actually Sits
Your body’s water isn’t just sloshing around freely. About two-thirds of it sits inside your cells, forming the medium where chemical reactions happen, nutrients get processed, and waste gets packaged for removal. The remaining one-third is outside your cells: in your blood plasma, in the fluid between tissues, and in smaller compartments like the fluid around your brain and spinal cord or the lubricating fluid inside your joints.
This distribution matters because your body tightly regulates the balance between these two compartments. Sodium and potassium concentrations on either side of cell membranes control how water moves in and out. When that balance gets disrupted, whether from illness, extreme sweating, or drinking far too much water at once, cells can swell or shrink in ways that cause real problems.
What All That Water Does
Water isn’t just filling space. It plays active roles in nearly every biological process your body runs. It dissolves and transports nutrients from your digestive tract to cells throughout the body. It carries waste products to your kidneys and liver for processing and removal. Every chemical reaction involved in breaking down food for energy takes place in a water-based environment.
Temperature regulation is another critical job. Water absorbs heat remarkably well, which is why your body uses it as a built-in cooling system. When you exercise or sit in the sun, your body pushes warm blood toward the skin and produces sweat. As that water evaporates, it pulls heat away from your body. Without enough water, this system falters and your core temperature can rise dangerously.
Water also cushions your joints, protects your spinal cord and brain, and keeps your eyes properly hydrated. It maintains cell volume, which is essential for cells to function and communicate normally.
How Much Water You Lose Each Day
Your body constantly loses water, even when you’re not sweating or exercising. About 400 milliliters per day evaporates directly through the skin without you noticing. Another 400 milliliters escapes through your lungs every time you exhale. These “insensible” losses alone account for roughly 800 milliliters, or about 3.5 cups, before you factor in urine, sweat, or digestion.
Total fluid output for a typical adult ranges from about 2 to 3 liters per day under normal conditions, and considerably more during exercise, hot weather, or illness. To keep the balance, studies suggest that 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 for men. That includes water from food, which typically contributes about 20% of your daily intake. Fruits, vegetables, soups, and even meats all contain significant water.
When Water Loss Becomes Dehydration
Dehydration is measured as a percentage of body weight lost through fluid. In children, losing as little as 3% of body weight in fluid is considered mild dehydration. At 6%, dehydration becomes moderate, with noticeable symptoms like dry mouth, reduced urination, and sunken eyes. Severe dehydration, at 9% or more, is a medical emergency. Infants have slightly more tolerance by percentage (mild up to 5%, severe at 10 to 15%) because they start with a higher baseline water content, but they also lose water faster relative to their size.
In adults, losing just 1 to 2% of body water can cause measurable drops in concentration, increase fatigue, and trigger headaches. By the time you feel thirsty, you’ve typically already lost about 1 to 2% of your body weight in water. Thirst is a lagging indicator, not an early warning system, which is why consistent fluid intake throughout the day matters more than drinking only when you feel dry.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that claims to measure your body water percentage, it’s using a method called bioelectrical impedance analysis. It sends a tiny electrical current through your body and measures resistance. Since water conducts electricity well and fat doesn’t, the scale estimates your water and lean mass from how easily the current travels. These devices are convenient but not highly precise. Studies in children have found that bioelectrical impedance underestimates total body water by about half a kilogram compared to more accurate laboratory methods.
The gold standard for measuring total body water involves drinking a small amount of water tagged with deuterium, a harmless heavy form of hydrogen. After a few hours, a blood or urine sample reveals how much the tagged water was diluted by your body’s existing water, giving a precise total volume. This approach is accurate but expensive, so it’s used mainly in research settings rather than routine checkups.