The average adult human body is about 55% to 60% water by weight. For a 70-kilogram (154-pound) person, that works out to roughly 38 to 42 liters of water, or about 10 to 11 gallons. But that number isn’t fixed. It shifts depending on your age, sex, and how much muscle versus fat you carry.
What Determines Your Body’s Water Content
The single biggest factor is body composition. Lean muscle tissue holds far more water than fat tissue does. This is the main reason adult men average around 60% water while adult women average closer to 55%: women tend to carry a higher proportion of body fat. Two people of the same weight can have meaningfully different total body water if one is more muscular than the other.
Age plays a major role too. Babies are born with the highest water content of any life stage, with water making up roughly 75% of a newborn’s body weight. Children still run high at 60% to 70%. By adulthood the number settles around 60% for men and 55% for women, then continues to drop. After age 60, total body water often falls to around 50%, driven by the gradual loss of muscle mass and an increase in body fat that comes with aging.
Where the Water Actually Sits
Your body’s water isn’t sloshing around freely. About 62% of it is locked inside your cells, where it serves as the medium for virtually every chemical reaction that keeps you alive. The remaining 38% is extracellular, meaning it’s in your blood plasma, the fluid between your cells, and specialized fluids like cerebrospinal fluid and the liquid in your joints.
Different organs hold dramatically different amounts of water. The lungs are the most water-rich organ at about 83%, which makes sense given that they need to stay moist to exchange oxygen and carbon dioxide. The brain and heart are both around 73%. Muscles and kidneys sit at roughly 79%. Skin, the body’s largest organ, is about 64% water. Even bones, which feel solid and dry, are still 31% water.
How Muscle and Fat Change the Equation
The relationship between body composition and water content is worth understanding because it explains so many of the individual differences. A lean, muscular person will have a higher total body water percentage than someone of the same weight with more body fat. This is true regardless of sex or age, though both of those factors influence body composition on average.
This also means that fitness level matters. Someone who builds significant muscle mass through exercise will carry a higher proportion of water than they did before, even at the same body weight. Conversely, gaining fat without gaining muscle will lower your body’s water percentage. It’s one reason why hydration recommendations aren’t perfectly one-size-fits-all.
How Much Water You Need Each Day
Maintaining that 55% to 60% balance requires a steady intake of water. The National Academies of Sciences sets the adequate intake for total water (from all beverages and food combined) at 3.7 liters per day for adult men and 2.7 liters per day for adult women. Those recommendations hold steady from age 19 through age 70 and beyond.
An important detail: “total water” includes water from food. Fruits, vegetables, soups, and other foods contribute roughly 20% of most people’s daily water intake. So the actual amount you need to drink is lower than those headline numbers suggest. For most men, that works out to about 3 liters (roughly 13 cups) of beverages, and for most women, about 2.2 liters (roughly 9 cups). Physical activity, heat, illness, and pregnancy or breastfeeding all push those needs higher.
Why Body Water Matters for Health
Water isn’t just filling space. Inside your cells, it’s the solvent that allows proteins to fold, enzymes to function, and nutrients to move where they’re needed. In your blood, it maintains the volume and pressure needed to deliver oxygen to tissues. In your kidneys, it’s the vehicle for filtering waste. In your joints, it provides cushioning. In your digestive tract, it keeps things moving.
Even mild dehydration, losing as little as 1% to 2% of your body water, can affect concentration, mood, and physical performance. Your body monitors its water balance tightly through thirst signals and hormone-driven adjustments in the kidneys. By the time you feel thirsty, you’re typically already slightly below your optimal hydration level, which is why consistent fluid intake throughout the day tends to work better than drinking large amounts at once.
For older adults, this becomes especially relevant. The decline in total body water with age means there’s less of a buffer against dehydration, while the thirst signal itself becomes less reliable. The combination makes older adults more vulnerable to dehydration even when they don’t feel particularly thirsty.