Water makes up about 55% to 60% of an adult’s total body weight. That number shifts depending on your age, sex, and how much body fat you carry, but for most adults, more than half of what you weigh is water.
The Average for Men and Women
Men and women carry different amounts of water relative to their body weight. The average adult man is about 60% water, while the average adult woman falls between 52% and 55%. The difference comes down to body composition: women typically carry a higher percentage of body fat, and fat tissue holds far less water than lean tissue. Muscle and other lean tissue is roughly 70% water by weight, while fat tissue contains only about 14% water. That sixfold difference in water content means your body composition has a bigger influence on your total water percentage than almost any other factor.
This is also why two people of the same sex and age can have noticeably different water percentages. A muscular person will have a higher percentage of body water than someone with more body fat, even if they weigh exactly the same.
How Body Water Changes With Age
You start life as mostly water and gradually dry out. A newborn’s body is about 75% water. By the time an infant reaches one year old, that drops to around 60%. Through childhood, body water percentage continues to shift gradually, reaching adult levels around age 12.
Adults maintain relatively stable water content through middle age, averaging around 55% of total body weight. After 65, both total body mass and water content decline. Older adults lose muscle mass over time, and since muscle is a major reservoir for water, total body water drops along with it. This is one reason older adults are more vulnerable to dehydration: they’re working with a smaller reserve to begin with.
Where the Water Actually Sits
Not all of your body water is sloshing around freely. About two-thirds of it, roughly 40% of your total body weight, is locked inside your cells. This is called intracellular fluid, and it’s where most of the chemical reactions that keep you alive take place. The remaining third, about 20% of body weight, lives outside your cells. This includes the liquid portion of your blood, the fluid that surrounds and cushions your cells, and smaller pockets like the fluid in your joints and eyes.
Clinicians sometimes call this the 60-40-20 rule: 60% of body weight is water, 40% is inside cells, 20% is outside cells. It’s a rough framework, but it holds up well for a typical adult man.
Water Content of Individual Organs
Some organs are far more water-dense than others. According to data compiled by the U.S. Geological Survey, the lungs are the most water-rich major organ at about 83%. The brain and heart are each roughly 73% water. Muscles and kidneys come in at 79%, while skin contains about 64%. Even your bones, which feel completely solid, are 31% water.
These numbers help explain why dehydration affects so many systems at once. When your body’s water supply drops, organs that depend heavily on water, especially the brain, feel it quickly. That’s why headaches, difficulty concentrating, and fatigue are among the earliest signs that you need fluids.
How Much Water You Lose Each Day
Your body cycles through a surprising amount of water every 24 hours. The biggest source of loss is urine, at roughly 1,500 milliliters per day. Another 900 milliliters evaporates through your skin and with every breath you exhale, a process you never notice because it happens continuously. Sweat accounts for about 50 milliliters on a sedentary day (far more if you exercise or live in a hot climate), and about 100 milliliters leaves in stool. That adds up to around 2,500 milliliters, or roughly 2.5 liters, lost daily under normal conditions.
Your body replaces this through drinking, food (most fruits and vegetables are 80% to 90% water), and a small amount of water generated internally when your body breaks down nutrients for energy.
What Happens When Levels Drop
Thirst is already a sign that you’ve dipped below your ideal water balance. Even mild dehydration can cause headaches, fatigue, and dizziness. At the mild stage, drinking water or an electrolyte-containing beverage usually resolves symptoms within five to ten minutes.
Moderate dehydration is more serious and can require intravenous fluids. Severe dehydration, if untreated, can be fatal. Older adults, young children, and people who are physically active or ill are at the highest risk because they either lose water faster or start with less of a buffer. Paying attention to urine color is one of the simplest ways to monitor hydration: pale yellow generally signals adequate water intake, while dark amber suggests you need more fluids.
How Scientists Measure Body Water
The gold standard for measuring total body water is a technique called isotope dilution. You drink a small, harmless dose of water that contains a traceable form of hydrogen. After a few hours, scientists analyze a saliva or blood sample to see how diluted the tracer has become, which reveals your total water volume with high precision. Consumer-level devices like bioelectrical impedance scales (the kind that send a small electrical current through your feet) estimate body water by measuring how easily electricity passes through your tissues. Water conducts electricity well, fat does not. These devices are convenient, but they’re less accurate than the isotope method, and readings can shift depending on your hydration status, when you last ate, and even whether you’re standing or lying down.