Water makes up about 50% to 65% of an adult’s body weight. For a 154-pound man, that works out to roughly 10.5 gallons (42 liters) of water carried around at all times. The exact percentage depends on your age, sex, and body composition, but water is by far the largest single component of the human body.
How Much Varies by Age and Sex
Newborns are the most water-dense humans. From birth to six months, a baby’s body averages about 74% water, with a range of 64% to 84%. That percentage drops quickly: by age one, it settles to around 60%, where it stays through childhood.
In adulthood, men carry more water than women. The average man’s body is about 60% water, while the average woman’s is closer to 52% to 55%. This gap comes down to body composition. Muscle tissue holds roughly six times more water per pound than fat tissue does. Since men tend to carry more muscle and less body fat, their overall water percentage runs higher. For the same reason, people with higher body fat percentages at any age tend to have a lower total water percentage.
As you age, total body water continues to decline. Men over 51 average about 56% water (ranging from 47% to 67%), while women over 51 average about 47% (ranging from 39% to 57%). This reflects both a gradual loss of muscle mass and shifts in body composition that come with aging.
Where All That Water Sits
Your body’s water isn’t evenly distributed. About two-thirds of it sits inside your cells, and the remaining third is outside them, either in the spaces between cells or in your blood. In that 154-pound man with 42 liters of total water, roughly 28 liters are locked inside cells, about 10.5 liters fill the spaces surrounding cells, and just 3.5 liters flow through the bloodstream. That blood volume, small as it sounds, is only about 8% of total body water.
This distribution matters because it affects how your body maintains blood pressure, delivers nutrients, and manages waste. The balance between water inside and outside cells is tightly regulated, and even small shifts can cause noticeable symptoms.
Water Content of Individual Organs
Some organs are far more water-rich than you might expect. Your lungs top the list at about 83% water, which makes sense given the moist membranes needed for gas exchange. Your kidneys come in at 79%. The brain and heart are both roughly 73% water. Even your bones contain water, though at much lower levels.
Muscle tissue is dramatically more hydrated than fat. Adipose (fat) tissue is only about 14% water, while the rest of the body’s lean mass averages around 80% water. This is why two people who weigh the same can have very different total body water: the person with more muscle is literally carrying more water.
What All That Water Does
Water isn’t just filler. It plays active roles in nearly every process that keeps you alive.
- Transport: Blood is mostly water, and it serves as the body’s primary delivery system for oxygen, nutrients, and hormones. It also carries waste products to the kidneys and lungs for removal.
- Temperature control: Water has a high heat capacity, meaning it absorbs a lot of energy before its temperature changes. This helps your body stay near its set point of 98.6°F (37°C) even when the environment around you is much hotter or colder. When you do overheat, sweating uses water evaporation to cool the skin.
- Chemical reactions: Nearly every metabolic reaction in your body takes place in water. Its neutral pH and ability to dissolve a wide range of substances make it the ideal medium for the thousands of enzymatic reactions happening at any given moment.
- Cushioning and lubrication: Cerebrospinal fluid protects the brain and spinal cord from impact. Synovial fluid lubricates joints. Pleural fluid around the lungs reduces friction during breathing. Mucus lines the digestive tract to ease food along. These are all primarily water.
How Much Water You Lose Each Day
Your body cycles through a surprising amount of water daily, even without heavy exercise. In a temperate climate, you lose about 450 milliliters through your skin as invisible perspiration (not counting visible sweat). Breathing releases another 250 to 350 milliliters as moisture in exhaled air. Your kidneys produce 1 to 2 liters of urine, and your digestive system accounts for roughly 200 milliliters lost through stool.
Add it all up and you’re losing somewhere around 2 to 3 liters per day under normal conditions. Exercise, heat, illness, and altitude all push that number higher. This constant outflow is why consistent fluid intake matters: you’re not filling a tank so much as maintaining a flow.
When Water Levels Drop Too Low
Dehydration is measured as a percentage of body weight lost to fluid. In children, losing up to 3% of body weight through fluid loss counts as mild dehydration. At 6%, dehydration is moderate, and at 9% it becomes severe. Infants are more vulnerable: moderate dehydration starts at 6% to 10% of body weight lost, and severe kicks in at 10% to 15%.
For adults, even a 1% to 2% drop in body water can cause noticeable symptoms like thirst, fatigue, and reduced concentration. By the time you feel thirsty, your body has already begun compensating by pulling water from less critical areas. The kidneys respond by concentrating urine, which is why darker urine is one of the earliest practical signals that you need more fluids.
Your body’s water content isn’t a fixed number. It fluctuates throughout the day as you drink, eat, sweat, and breathe. But the narrow range your body works to maintain, roughly 50% to 65% of your weight, reflects just how central water is to keeping every system running.