Water makes up about 60% of an adult male’s body weight and roughly 55% of an adult female’s. For a 180-pound man, that translates to about 108 pounds of water. These numbers shift considerably depending on your age, body composition, and how much muscle versus fat you carry.
How Body Water Changes With Age
You start life as mostly water. Newborns are about 75% water by weight, which drops to around 60% by the time they reach their first birthday. From there, body water gradually decreases through childhood, settling into adult ranges around age 12.
Adults between 13 and 64 average around 55% to 60% total body water, with the variation largely explained by sex and body composition. After 65, both total body mass and water content decline further. Older adults tend to carry less water partly because they lose muscle mass and partly because the body’s ability to conserve water becomes less efficient with age.
Why Men and Women Differ
The gap between men and women comes down to fat tissue. Women naturally carry a higher proportion of body fat, and fat holds dramatically less water than muscle. Lean tissue is about 70% water, while adipose (fat) tissue is only about 14 to 20% water. That means lean tissue holds roughly six times more water per pound than fat does. So two people who weigh the same can have very different total body water levels depending on their ratio of muscle to fat.
This is also why athletes and people with more muscle mass tend to have body water percentages above 60%, while someone with a higher body fat percentage may fall well below 55%.
Where All That Water Sits
Your body’s water isn’t evenly distributed. About two-thirds of it is inside your cells, where it serves as the medium for nearly every chemical reaction that keeps you alive. The remaining one-third sits outside the cells: in your blood, in the fluid between tissues, and in smaller compartments like cerebrospinal fluid and the fluid inside your eyes.
This is sometimes called the 60-40-20 rule. If 60% of your body weight is water, roughly 40% is inside cells and 20% is outside them. Your body works constantly to maintain this balance. When the ratio shifts, whether from dehydration, illness, or fluid overload, you feel it quickly.
Water Content of Individual Organs
Some organs are far more water-dense than others. Your lungs are the most water-rich major organ at about 83%. The brain and heart are each around 73%, and the kidneys come in at about 79%. Even your bones contain some water, though far less than soft tissue. These numbers help explain why dehydration can affect so many systems at once: your brain, heart, and kidneys all depend heavily on adequate water to function.
How Much Water You Cycle Through Daily
Your body doesn’t just store water. It continuously loses and replaces it. Men take in an average of 3.0 liters of water per day from beverages and food combined, while women average about 2.5 liters. Those numbers drop slightly in older adults, with men in their 70s averaging 2.8 liters and women of the same age averaging 2.3 liters.
On the output side, urine accounts for the largest share at about 2.2 liters per day for both men and women. The rest leaves through breathing, sweat, skin evaporation, and stool. Your body adjusts this turnover constantly, producing more or less urine depending on how much fluid you’re taking in and how much you’re losing through sweat and respiration.
What Dehydration Looks Like by the Numbers
Dehydration is measured as a percentage of body weight lost through fluid. In children, losing just 3% of body weight to fluid loss is considered mild dehydration. At 6%, it’s moderate, and 9% or above is severe. Infants can tolerate slightly higher percentages before reaching the same severity levels, with mild dehydration classified as up to 5% body weight loss and severe at 10 to 15%.
For adults, losing even 1 to 2% of body weight in water is enough to cause noticeable thirst, fatigue, and reduced concentration. By the time you feel genuinely thirsty, you’re typically already mildly dehydrated. Physical performance starts declining at around 2%, and cognitive effects become more pronounced beyond that point.
How Body Water Is Measured
If you’ve ever stepped on a smart scale at home, it likely estimated your body water using bioelectrical impedance. This method sends a small electrical current through your body: water conducts electricity well, so the device estimates total water based on how easily the current passes through. It’s convenient but only moderately accurate, since hydration status, recent meals, and even skin temperature can skew results.
The gold standard in research settings is deuterium dilution. You drink a small, harmless dose of “heavy water” (water made with a heavier form of hydrogen), wait a few hours for it to distribute evenly, then provide a breath or blood sample. By measuring how much the deuterium was diluted, researchers can calculate your total body water with high precision. Skinfold thickness measurements offer another rough estimate by assessing body fat, since fat and water content are inversely related.
For most people, the practical takeaway is simpler than any measurement technique: if you’re a healthy adult, somewhere between 50% and 65% of your weight is water, and staying well-hydrated keeps that number in a range where your body functions best.