Water makes up roughly 55% to 60% of an adult’s total body weight, though the exact number depends on your age, sex, and body composition. For a 154-pound person, that translates to about 10 gallons of water distributed across every organ, tissue, and cell.
Average Body Water by Age and Sex
Men generally carry a higher percentage of water than women because they tend to have more muscle mass (muscle is about 76% water, while fat tissue holds considerably less). For adult men aged 19 to 50, the average is around 59% of body weight, with a normal range of 43% to 73%. For women in the same age group, the average drops to about 50%, ranging from 41% to 60%.
These numbers shift as you age. Men over 51 average about 56%, while women over 51 average around 47%. The decline happens because older adults naturally lose muscle mass and may gain proportionally more fat tissue, both of which lower total body water. Hormonal changes after menopause also contribute to the steeper drop in women.
At the other end of the spectrum, newborns are about 75% water by weight. By the time a baby reaches one year old, that figure settles to around 60%, and it gradually approaches adult levels by age 12.
Where the Water Actually Sits
Not all of your body’s water is in one place. About two-thirds of it, roughly 25 liters (seven gallons) in an average-sized adult male, is inside your cells. This intracellular fluid is where most of the chemical reactions that keep you alive take place.
The remaining third sits outside your cells. This extracellular fluid includes the liquid portion of your blood (plasma), which accounts for about 20% of that outer compartment, and interstitial fluid, the thin layer of water that bathes and cushions every cell in your body. Together, these two compartments form a carefully balanced system. Your kidneys, hormones, and electrolytes work constantly to keep the ratio stable.
Water Content of Individual Organs
Some organs are far more water-dense than others, and the differences are striking. Your lungs top the list at about 83% water, which makes sense given their role in gas exchange across moist surfaces. The brain and heart are each about 73% water. Muscles and kidneys come in at roughly 79%, while skin, the body’s largest organ, is around 64%.
Bones are the driest major tissue, but even they contain a meaningful amount of water. These organ-level differences explain why dehydration can affect so many systems at once. A drop in total body water ripples through your brain (concentration and mood), your muscles (strength and endurance), and your kidneys (waste filtering) simultaneously.
Why Body Composition Matters So Much
Two people of the same age and height can have meaningfully different body water percentages based solely on how much muscle versus fat they carry. Muscle tissue is about 76% water, making it one of the most water-rich tissues in the body. Fat tissue holds far less. So a lean, muscular person will have a higher total body water percentage than someone of the same weight with more body fat.
This is the main reason men average higher percentages than women, and it’s also why athletes often test above the standard ranges. It’s worth keeping in mind if you ever see your body water percentage on a smart scale or body composition analysis. A number on the lower end of the range doesn’t necessarily mean you’re dehydrated. It may simply reflect your ratio of fat to lean tissue.
How Much Your Water Weight Fluctuates
Your body’s water content isn’t fixed from hour to hour. Total body water can fluctuate by up to 5% in a single day, which translates to roughly 1 to 5 pounds of weight change. This is why your scale can read differently in the morning versus the evening, or the day after a salty meal compared to a normal day.
Several things drive these swings. Sodium intake causes your body to retain more water in the extracellular space. Carbohydrate intake matters too, because your body stores carbs alongside water in your muscles and liver. Hormonal cycles, particularly in women, can shift fluid balance over a period of days. Exercise causes acute water loss through sweat, but the body compensates quickly once you rehydrate. None of these short-term fluctuations are cause for concern. They’re a normal part of how your body manages fluid balance.
How Much Water You Need to Maintain Balance
Your body loses water continuously through breathing, sweating, and urine, so it needs a steady supply coming in. Current guidelines suggest that the average healthy adult needs about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, with the higher end typically applying to men. That total includes water from all sources: plain drinking water, other beverages, and the water naturally present in food, which typically accounts for about 20% of your daily intake.
Fruits, vegetables, soups, and yogurt all contribute meaningful amounts of water. Your actual needs will vary with climate, physical activity level, and overall health. Thirst is a reasonably good guide for most healthy adults, though it becomes less reliable with age, which is one reason older adults are more vulnerable to dehydration despite already having lower baseline body water.