The average adult human body is about 55 to 60% water by weight. For a 70-kilogram (154-pound) person, that translates to roughly 38 to 42 liters of water, or about 10 to 11 gallons. But that number shifts significantly depending on your age, sex, and body composition.
How Water Content Changes With Age and Sex
Babies are the most water-dense humans. Young children carry about 62% of their body weight as water, with little difference between boys and girls. That changes at puberty, when body composition starts to diverge. Adult males typically hold 55 to 60% water, while adult females carry 50 to 55%. The difference comes down to body fat: fat tissue contains less water than muscle, and women on average carry a higher percentage of body fat.
After age 60, water content drops again in both sexes. Men settle around 57%, and women closer to 50%. This gradual decline is one reason older adults are more vulnerable to dehydration, even mild heat or a skipped glass of water can tip the balance faster than it would in a younger person.
Where Water Lives Inside Your Body
Not all of that water sits in one place. About 62% of your total body water is inside your cells, where it serves as the medium for virtually every chemical reaction keeping you alive. The remaining 38% is outside cells: in your blood plasma, in the fluid between tissues, and in smaller reservoirs like spinal fluid and the moisture lining your lungs and digestive tract.
This ratio matters because your body works hard to maintain it. When you’re dehydrated, water shifts between compartments to keep critical systems running. When the balance tips too far, cells either swell or shrink, and both directions cause problems.
Water Content by Organ
Some organs are far more water-rich than others. According to data compiled by the U.S. Geological Survey, the lungs top the list at about 83% water, which makes sense given the moist surfaces needed for gas exchange. The brain and heart are both around 73%. Muscles and kidneys sit at 79%, while skin is about 64%. Even bones, which feel solid and dry, are 31% water.
These numbers help explain why dehydration affects thinking and physical performance so quickly. The organs that demand the most water are the same ones responsible for cognition, circulation, and movement.
How Much Water You Lose Each Day
Your body cycles through a surprising volume of water every 24 hours. The biggest route of loss is urine, at roughly 1,500 milliliters (about 6 cups) per day in a typical adult. Another 900 milliliters leaves through your skin and lungs without you noticing. This “insensible loss” happens with every exhale and through constant low-level evaporation from your skin, even when you’re not sweating. Sweat itself accounts for about 50 milliliters on a sedentary day, though that number can spike dramatically during exercise or heat exposure. A small amount, around 100 milliliters, leaves through your stool.
Add it all up and you’re losing roughly 2,500 milliliters (about 2.5 liters) of water daily under normal, resting conditions. Physical activity, hot weather, illness, or altitude can push that figure much higher.
How Your Body Replaces That Water
Most replacement comes from drinking fluids and eating food. Fruits, vegetables, soups, and even bread all contribute water. But your body also manufactures a small amount internally. When cells break down carbohydrates, fats, and proteins for energy, water is a byproduct of the chemical reactions. This “metabolic water” can reach about 300 milliliters per day, covering roughly 10% of your daily needs. The other 90% has to come from what you eat and drink.
The balance between intake and output is what determines your hydration status. Your kidneys are the primary regulators, adjusting how much water they retain or release in urine. When you’re well hydrated, urine is pale and plentiful. When water is scarce, your kidneys concentrate it, producing less urine with a darker color. Thirst typically kicks in after you’ve already lost about 1 to 2% of your body weight in water, which is why relying on thirst alone can leave you mildly behind on fluid intake, especially during exercise.
Why Body Composition Changes the Number
Two people of the same weight can have very different water totals. Muscle tissue is about 79% water, while fat tissue holds significantly less. Someone with more muscle mass will carry a higher percentage of body water than someone with more fat mass, even if they weigh exactly the same. This is the main reason athletes and lean individuals test at the higher end of the range, while people with higher body fat percentages fall toward the lower end.
It also explains why hydration advice isn’t truly one-size-fits-all. A muscular 200-pound person has more water to maintain, and more to lose during exertion, than a 200-pound person with a higher fat percentage. Individual variation in total body water is one of the reasons blanket recommendations like “eight glasses a day” have never been strongly supported by evidence. Your actual needs depend on your size, composition, activity level, climate, and diet.