The average adult human body is about 55 to 60 percent water. Men typically carry around 60 percent, while women average closer to 50 to 55 percent, largely because women tend to have a higher proportion of body fat, which holds less water than other tissues. That means a 180-pound man is carrying roughly 11 gallons of water at any given moment.
How Body Water Changes With Age
You start life as mostly water, and that percentage steadily drops over the decades. Newborns average about 74 percent water, with a range as wide as 64 to 84 percent. By the time a baby reaches six months, that number settles to around 60 percent, where it stays through childhood.
The decline continues into older adulthood. Men over 51 average about 56 percent water, down from 59 percent in younger adults. Women over 51 average around 47 percent, down from 50 percent. This gradual loss is one reason older adults are more vulnerable to dehydration and its effects on kidney function, blood pressure, and cognitive sharpness.
Where the Water Actually Sits
Not all of your body’s water sloshes around freely. About two-thirds of it is locked inside your cells, forming what physiologists call intracellular fluid. This is the water your cells use for chemical reactions, energy production, and maintaining their structure. The remaining third sits outside cells: in your blood plasma, in the fluid between tissues, and in specialized fluids like cerebrospinal fluid and the liquid inside your eyes.
Blood itself is about 80 percent water. The plasma portion, the yellowish liquid that carries red and white blood cells through your veins, is 90 percent water. That’s why even mild dehydration can thicken your blood and force your heart to work harder.
Some Organs Hold Far More Water Than Others
Your body’s water isn’t evenly distributed. Muscle tissue is about 76 percent water, making it one of the most water-dense tissues in your body. This is partly why muscular people tend to have a higher total body water percentage than people with less lean mass, and why losing muscle with age contributes to the overall decline in body water.
Fat tissue, by contrast, contains significantly less water. This single difference explains most of the gap between men and women: because women on average carry more body fat relative to lean mass, their overall water percentage is lower. It also means two people of the same age and sex can differ by 15 to 20 percentage points depending on body composition. Among adult men aged 19 to 50, total body water ranges from as low as 43 percent to as high as 73 percent.
What All That Water Does
Water isn’t just filling space. It performs a long list of jobs that keep you alive and functional:
- 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. Without adequate water, this cooling system fails quickly.
- Nutrient and oxygen transport. Water in your blood plasma carries dissolved nutrients and oxygen to every cell in your body and hauls waste products back to the kidneys and liver for removal.
- Joint lubrication. The fluid in and around your joints is primarily water. It reduces friction between bones and absorbs shock during movement.
- Waste removal. Your kidneys filter about 120 to 150 quarts of fluid per day, relying on water to flush out toxins and metabolic byproducts through urine.
- Tissue protection. Water moistens the tissues in your eyes, nose, and mouth, and it cushions sensitive organs like the brain and spinal cord.
- Chemical reactions. Nearly every metabolic process in your body, from digesting food to building new proteins, takes place in a water-based solution inside your cells.
Why Body Composition Matters More Than Weight
Because muscle and fat hold such different amounts of water, your body composition is a better predictor of hydration status than your weight alone. Two people who weigh 160 pounds can have very different total body water levels if one is lean and muscular while the other carries more fat. Athletes and people with higher muscle mass routinely test above 60 percent body water, while someone with a higher body fat percentage may fall below 45 percent.
This also has practical implications for how quickly dehydration affects you. Someone with more lean mass has a larger water reserve to draw from, while someone with less lean mass may feel the effects of fluid loss sooner. Age compounds this: as you lose muscle over the decades, your buffer against dehydration shrinks alongside it.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that reports body water percentage, it was using a technique called bioelectrical impedance. The scale sends a tiny electrical current through your body and measures how quickly it travels. Water conducts electricity well, so the faster the signal moves, the more water you’re carrying. These consumer devices give a rough estimate but can be thrown off by hydration status, recent meals, and even skin temperature.
In clinical and research settings, the gold standard involves drinking a small, measured dose of “heavy water,” a form of water made with a heavier version of hydrogen. Researchers then test a sample of saliva, blood, or urine to see how much the heavy water was diluted by the body’s existing water supply. From that dilution ratio, they calculate total body water with high precision. One large study using this method found that total body water represented about 74 percent of fat-free mass, confirming that lean tissue is overwhelmingly water.