The average adult body is roughly 55% to 60% water by weight. For a 155-pound man, that works out to about 10 to 11 gallons of water. For a 130-pound woman, it’s closer to 7 to 8 gallons. These numbers shift based on your age, body composition, and sex, but water is by far the largest single component of the human body.
Average Water Percentage by Sex and Age
Adult men carry about 60% of their body weight as water, while adult women average 52% to 55%. The difference comes down to body composition: fat tissue holds far less water than lean tissue, and women typically carry a higher proportion of body fat. About 73% of lean body mass is water, while fat tissue can contain as little as 17% water, though that number varies widely depending on the type of fat and where it’s stored.
Newborns are the most water-dense humans, at roughly 75% water by body weight. That percentage gradually declines through childhood and adolescence as body fat increases and tissues mature. By older adulthood (65 and beyond), both total body mass and water content drop further, partly because muscle mass decreases and partly because the body becomes less efficient at maintaining fluid balance. This is one reason older adults are more vulnerable to dehydration.
Where the Water Actually Sits
Your body’s water isn’t sloshing around freely. About two-thirds of it sits inside your cells, in what physiologists call the intracellular compartment. The remaining third exists outside cells: in your blood plasma, in the fluid between cells, and in specialized spaces like the fluid around your brain and spinal cord, the fluid inside your eyes, and the lining of your joints. The ratio between these two compartments stays remarkably consistent in healthy people, with the outside-the-cell volume running about 56% of the inside-the-cell volume.
This distribution matters because your body constantly shuttles water between compartments to maintain blood pressure, deliver nutrients, and remove waste. When you’re dehydrated, water shifts out of cells to keep blood volume stable. When you consume excess sodium, your body retains extra water in the spaces outside cells, which is what causes bloating and puffiness.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. Some organs are surprisingly water-rich:
- Lungs: about 83% water, the highest of any major organ
- Muscles and kidneys: about 79% water
- Brain and heart: about 73% water
- Skin: about 64% water
- Bones: about 31% water, even though they feel completely solid
Your muscles are your largest water reservoir simply because of their size. Someone with more muscle mass holds more total water than someone of the same weight with more body fat. This is the single biggest reason body water percentages vary so much from person to person.
How Body Composition Changes Your Numbers
Two people who weigh exactly the same can have very different total body water. A lean, muscular 170-pound person might be 63% water, while a 170-pound person with higher body fat might be closer to 52%. The lean tissue assumption of 73% water is so reliable that medical imaging techniques actually use it as a constant when calculating body fat percentage.
Fat tissue is the wildcard. Its water content ranges from about 17% to as high as 84% depending on the type of adipose tissue and how metabolically active it is. But on average, fat holds dramatically less water than muscle, which is why athletes and people with lower body fat percentages consistently test higher for total body water. This also explains why body water percentage can serve as a rough proxy for fitness level on body composition scales.
Calculating Your Own Total Body Water
If you want a more personalized estimate, the Watson Formula is the most widely used clinical calculation. For men, it factors in age, height in centimeters, and weight in kilograms. For women, it uses height and weight. The output is your estimated total body water in liters. Since one liter of water weighs one kilogram, you can divide the result by your body weight in kilograms to get your percentage.
As a quick rule of thumb: multiply your body weight in pounds by 0.6 if you’re male or 0.5 if you’re female, then divide by 2.2 to convert to liters. A 180-pound man gets roughly 49 liters (about 13 gallons). A 140-pound woman gets roughly 32 liters (about 8.5 gallons). These are ballpark figures that assume average body composition for your sex.
What Makes Your Body Hold More or Less Water
Your total body water fluctuates by one to four pounds on any given day. Sodium is the primary driver of short-term water retention. When you eat a salty meal, your kidneys respond by holding onto extra water to keep sodium concentration in your blood within a narrow range. Chloride, sodium’s usual partner in table salt, plays a supporting role in this same process.
Glycogen, the stored form of carbohydrate in your muscles and liver, also binds water. Every gram of glycogen holds roughly three grams of water alongside it. This is why people on very low-carb diets lose several pounds of water weight in the first week: they’re depleting glycogen stores and releasing the water that came with them.
Hormones regulate the bigger picture. Your brain constantly monitors blood concentration and blood pressure, then signals the kidneys to either conserve or release water. When you’re dehydrated, hormones tell your kidneys to concentrate your urine and pull water back into the bloodstream. When you’ve had plenty to drink, the signal flips and your kidneys let more water pass through. This system is why healthy people maintain a stable body water percentage despite wide variation in daily fluid intake.
Menstrual cycle hormones also affect water retention, which is why many women notice a few pounds of fluctuation at predictable points in their cycle. Heat, altitude, exercise intensity, and certain medications can all shift the balance as well, though the body typically corrects these within hours to days.