The average adult human body is about 60% water by weight. For a 155-pound person, that works out to roughly 10 to 11 gallons of water. But that number isn’t the same for everyone. It shifts based on your age, sex, and body composition, and it changes throughout your lifetime.
How Water Percentage Varies by Age and Sex
Newborns are the most water-dense humans. A full-term baby is about 74% water, and premature infants can be as high as 90% water at 26 weeks gestation. That percentage drops quickly, falling roughly 1.3% per week in the early days of life.
By adulthood, the numbers settle into a range that differs between men and women. The average man is about 60% water, while the average woman is 52 to 55%. The difference comes down to body composition: fat tissue holds much less water than muscle tissue. Since women typically carry a higher percentage of body fat, their overall water percentage is lower. For the same reason, a very muscular person of either sex will carry proportionally more water than someone with more body fat.
In older adults, total body water drops further. You naturally lose muscle mass as you age, and your kidneys become somewhat less efficient at conserving water. This is one reason dehydration becomes a bigger risk later in life.
Water Content of Individual Organs
Not all parts of the body hold water equally. The lungs are the most water-rich organ at 83%. Muscles and kidneys come in at 79%, while the brain and heart are each about 73%. The liver sits at 71%. Even bones, which seem dry and solid, are 31% water.
Blood is about 80% water overall. The liquid portion of blood, called plasma, is 90% water. Plasma acts as the river system that carries nutrients, hormones, and waste products throughout your body.
Where All That Water Actually Sits
Your body’s water is divided into two main compartments. About two-thirds of it sits inside your cells, forming the fluid where most of your body’s chemical reactions take place. The remaining third is outside your cells: in your blood, in the fluid between tissues, and in smaller reservoirs like your spinal fluid and the liquid inside your eyes and joints.
Doctors sometimes call this the 60-40-20 rule. If total body water is 60% of your weight, roughly 40% is inside cells and 20% is outside them. Your body works constantly to keep this ratio stable, because even small shifts can affect how your cells function.
What Water Does in Your Body
Water isn’t just filling space. It serves as the solvent for nearly every chemical process that keeps you alive. The carbohydrates and proteins you eat are broken down and carried through your bloodstream in water. Your joints stay cushioned because water is a key component of the fluid between them. And your body regulates its temperature through water, primarily by sweating and by releasing moisture when you breathe out.
Your body even manufactures a small amount of water on its own. When cells break down fats, carbohydrates, and proteins for energy, water is produced as a byproduct. This “metabolic water” accounts for up to 300 milliliters per day, roughly a cup and a quarter. That covers about 10% of your daily water needs. The rest has to come from drinking fluids and eating water-containing foods.
How Your Body Maintains Water Balance
Your brain and kidneys run a tight feedback loop to keep water levels steady. When sensors in your brain detect that your blood is becoming too concentrated (a sign you need more water), they trigger the release of a hormone that tells your kidneys to hold onto water. The kidneys respond by inserting tiny water channels into their filtering tubes, allowing water to be reabsorbed back into your bloodstream instead of lost as urine. Your urine becomes more concentrated and lower in volume as a result.
Once your blood returns to a normal concentration, the brain dials the signal back down, the kidneys let more water pass through, and your urine becomes lighter and more dilute. This cycle runs continuously. When the system is working well, your body can adjust water retention within minutes of drinking a glass of water or sweating through a workout. When the system fails, either through disease, certain medications, or extreme dehydration, the consequences can be serious and rapid.
Why Body Composition Matters More Than Weight
Two people who weigh exactly the same can have very different amounts of total body water. The key variable is the ratio of muscle to fat. Muscle tissue is about 79% water, while fat tissue holds considerably less. This is why an athlete and a sedentary person of the same weight will have different water percentages, and why broad statements like “the body is 60% water” are averages rather than universal truths.
If you’re curious about your own total body water, the most accessible method is bioelectrical impedance analysis. It works by sending a tiny, painless electrical current through your body and measuring resistance. Since water conducts electricity well and fat does not, the device can estimate how much of your weight is water. Many home scales now include this feature, though their accuracy varies. The gold standard in clinical settings is isotope dilution, where you drink a small amount of water tagged with a traceable molecule, and researchers measure how it distributes through your body over several hours. This method is highly accurate but impractical outside of a lab.
For most people, knowing the general range is enough. If you’re an adult man, expect roughly 55 to 65% water. If you’re an adult woman, 45 to 55% is typical. Higher muscle mass pushes you toward the upper end, more body fat pulls you toward the lower end, and aging gradually moves the number down over decades.