The human body is roughly 50% to 65% water by weight in the average adult. That range isn’t a single fixed number because your exact percentage depends on your age, sex, and body composition. A lean, muscular 25-year-old man might be closer to 65%, while an older woman with more body fat might be closer to 50%.
How Water Content Changes With Age
You start life as mostly water. At birth, about 70% of a baby’s body weight is water, and that proportion stays high through early childhood. As you grow and gain more fat tissue relative to lean tissue, the percentage gradually decreases. By adulthood, most people settle into the 50% to 65% range, where total body water stays relatively stable through a large portion of adulthood.
In older adults, the percentage drops further. This happens because aging typically brings an increase in body fat and a loss of muscle mass. Since fat tissue holds significantly less water than muscle, the overall proportion shifts downward. This is one reason older adults are more vulnerable to dehydration: they’re working with a smaller water reserve to begin with.
Why Men and Women Differ
The average man carries about 60% of his body weight as water, while the average woman is closer to 52% to 55%. The difference comes down to body composition. Women naturally carry a higher percentage of body fat than men, and fat tissue contains far less water than lean tissue. Muscle and other lean tissues are about 79% water, while fat tissue can hold as little as 17% water, though that number varies widely depending on the type of fat.
This same principle applies regardless of sex: the more muscle you carry relative to fat, the higher your body water percentage will be. Two people of the same weight can have noticeably different water percentages if one is more muscular.
Water Content of Individual Organs
Water isn’t distributed evenly throughout your body. Some organs are far more water-dense than others:
- Lungs: 83%
- Muscles and kidneys: 79%
- Brain and heart: 73%
- Skin: 64%
- Bones: 31%
Even your skeleton, which feels about as dry as a body part can get, is nearly one-third water. The lungs top the list because they need a thin layer of moisture to facilitate gas exchange with every breath you take. Muscles and kidneys rank high because both are metabolically active tissues that rely on water for nearly every chemical reaction happening inside their cells.
What All That Water Actually Does
Water isn’t just filling space. It plays active roles in almost every system in your body. It regulates your temperature through sweating and blood flow to the skin. It lubricates your joints, cushioning them against the impact of movement. It carries nutrients and oxygen to your cells and flushes waste products through your kidneys and liver. It moistens the tissues in your eyes, nose, and mouth. And it dissolves minerals and other nutrients so your body can actually absorb and use them.
At the cellular level, water is the medium in which virtually all of your body’s chemistry takes place. Proteins fold, enzymes catalyze reactions, and cells divide, all in a water-based environment. Without adequate water, these processes slow down or stop entirely, which is why even mild dehydration can affect your energy, concentration, and physical performance.
How Your Body Maintains Its Water Balance
Your body doesn’t leave something this important to chance. The kidneys are the primary regulators, and they’re controlled by a hormone called ADH (anti-diuretic hormone), which is released by the brain. When your blood becomes too concentrated, meaning you’re running low on water, specialized sensors in the brain detect the change and trigger ADH release. ADH tells the kidneys to reabsorb more water instead of excreting it, so your urine becomes more concentrated and you conserve fluid.
When you’re well-hydrated, ADH drops, and the kidneys let more water pass through into your urine. This is why your urine is pale when you’ve been drinking plenty of fluids and dark when you haven’t.
A second system works alongside ADH. Your adrenal glands produce a hormone called aldosterone, which controls how much sodium your kidneys retain. Since water follows sodium, holding onto more sodium means holding onto more water. This system kicks in when your blood pressure drops or your blood becomes too concentrated, helping restore fluid volume. Together, these two hormonal systems keep your body water within a remarkably tight range despite wide day-to-day variations in how much you drink and sweat.
How Body Water Is Measured
If you’ve ever wondered how scientists arrived at these numbers, the gold standard is a technique called isotope dilution. You drink a small, harmless dose of water labeled with a traceable form of hydrogen (deuterium). After a few hours, once the labeled water has mixed thoroughly with all the water in your body, a saliva or urine sample reveals how diluted the tracer has become. The more diluted it is, the more total body water you have. The whole process takes about five hours with saliva sampling, though urine-based methods can stretch over one to two weeks for greater precision.
A faster, more practical option is bioelectrical impedance analysis, which sends a tiny electrical current through your body. Water conducts electricity well, so the more water you have, the less resistance the current encounters. Many smart scales and body composition devices use this method, though it’s less accurate than isotope dilution and can be thrown off by your hydration status at the moment of measurement.
Ethnicity and Individual Variation
Research published in Kidney International, drawing from over 1,600 adults aged 18 to 90, found that Black adults had higher average total body water than white adults across all age groups. The 75th percentile for white participants roughly matched the median for Black participants at most ages. These differences likely reflect variations in average body composition, including muscle mass and fat distribution, between populations. Individual variation within any group is significant, typically ranging 3 to 5 liters above or below predicted values.