The average adult body is about 55% to 60% water by weight. For a 155-pound person, that translates to roughly 10 to 11 gallons of water carried at any given moment. The exact amount varies depending on your age, sex, and body composition, but water is by far the largest single component of the human body.
Average Water Content by Age and Sex
Adult men carry about 60% of their body weight as water, while adult women average 52% to 55%. The difference comes down to body fat: fat tissue holds much less water than lean tissue, and women on average carry a higher proportion of body fat. A 180-pound man holds roughly 12 gallons of water, while a 140-pound woman holds closer to 8 or 9 gallons.
Newborns are the most water-dense humans, at about 75% water by total weight. That percentage drops gradually through childhood, reaching adult levels around age 12. At the other end of life, total body water falls again in adults over 65 as both muscle mass and overall body weight decline. An older adult may carry closer to 50% of their weight as water.
Where the Water Actually Sits
About two-thirds of your body water is inside your cells. This intracellular water is where most of the chemistry of life happens: energy production, protein building, waste processing. The remaining third sits outside cells, split between the fluid portion of your blood (plasma), the liquid between cells, and smaller reserves like cerebrospinal fluid and the fluid in your eyes and joints.
The ratio between these two compartments stays remarkably stable in healthy people, with the outside-to-inside ratio hovering around 0.56. When that ratio shifts, whether from dehydration, heart failure, or kidney disease, it signals that something has gone wrong with the body’s fluid regulation.
Water Content of Individual Organs
Not every part of your body holds water equally. Your lungs are the most water-rich major organ at about 83% water, which makes sense given their role in gas exchange across moist membranes. The brain and heart are each about 73% water. Muscles and kidneys come in at 79%, and even your skin is 64% water.
Bones might seem dry, but they’re still 31% water. Muscle tissue, at 76% water, is one reason body composition matters so much to your total water volume. A muscular person of the same weight as a less muscular person will carry noticeably more water. This also means that losing muscle, whether from aging, illness, or inactivity, reduces total body water and can make you more vulnerable to dehydration.
How Your Body Cycles Through Water Daily
Your body doesn’t just hold water passively. It turns over roughly 4% of its total water supply every day. For a 155-pound adult, that’s about 2,500 to 3,000 milliliters (roughly 10 to 12 cups) moving in and out daily.
On the intake side, most water comes from beverages, but solid foods contribute about 20% of your daily water, or roughly 700 to 800 milliliters. Your body also generates a small amount of water internally, about 250 milliliters per day, as a byproduct of breaking down food for energy.
On the output side, half of your daily water loss comes from sources you barely notice: evaporation through your skin and moisture in exhaled air. These “insensible losses” happen constantly, even when you’re not sweating. The kidneys handle the rest, adjusting how much water leaves in your urine based on what your body needs to keep fluid levels stable.
How Your Body Regulates Its Water Supply
Your kidneys are the main control point. They can produce concentrated urine to conserve water when you’re dehydrated, or dilute urine to flush excess water when you’ve had too much. The signal to do one or the other comes from a hormone released by the brain called antidiuretic hormone, or ADH. When sensors in the brain detect that your blood is becoming too concentrated, ADH surges and tells the kidneys to reabsorb more water. When your blood is dilute, ADH drops and the kidneys let water pass through into urine.
A second system focuses on sodium, which directly influences how much water your body retains. The adrenal glands produce a hormone called aldosterone that tells the kidneys to hold onto sodium. Because water follows sodium, retaining more sodium means retaining more water. This system activates when blood pressure drops or when the body senses it needs more fluid volume. It’s the reason high-sodium meals can leave you feeling puffy and heavier the next morning: your body held onto extra water to balance the salt.
These two systems work in tandem, with additional input from stretch receptors in the heart and major arteries that sense blood volume and pressure. The result is a tightly controlled system that keeps your total body water within a narrow range despite wide variation in how much you drink, eat, sweat, and breathe on any given day.
What Affects Your Personal Water Volume
Your total body water is shaped primarily by three factors: how much muscle you carry, how much fat you carry, and your age. Since muscle is 76% water and fat tissue holds significantly less, two people at the same weight can have very different total water volumes depending on their body composition. Athletes and people with higher muscle mass will sit at the higher end of the range, while people with higher body fat percentages will sit lower.
Hydration status itself only shifts total body water by a small amount in healthy people, because the kidney and hormone systems described above correct imbalances quickly. You can measure your total body water with a method called bioelectrical impedance analysis, which sends a small electrical current through your body. Water conducts electricity well, so the device estimates your water volume based on how easily the current passes through. Many body composition scales at gyms and clinics use this technology. The gold standard for research is isotope dilution, where you drink water labeled with a traceable marker and measure how it disperses, but this is rarely done outside of research settings.
For most people, the practical takeaway is straightforward: if you weigh 150 pounds, roughly 80 to 90 pounds of that is water. It fills every cell, cushions your organs, carries nutrients through your blood, and regulates your temperature. It’s the medium in which virtually all of your body’s chemistry takes place.