The average adult body is roughly 50% to 60% water by weight. For a 155-pound person, that translates to about 10 to 11 gallons of water spread across every organ, tissue, and cell. The exact percentage depends on your age, sex, and how much body fat you carry.
Body Water by Age and Sex
Newborns are the most water-dense humans. From birth to six months, an infant’s body averages about 74% water, with a range of 64% to 84%. That proportion drops steadily with age as body composition changes.
Adult men between 19 and 50 average around 59% water, while women in the same age range average about 50%. The gap comes down largely to body fat: women typically carry a higher percentage of fat tissue, which holds far less water than muscle (more on that below). After age 50, both numbers dip a few points. Men average about 56% and women about 47%, reflecting the gradual loss of lean tissue that comes with aging.
Why Fat and Muscle Matter So Much
Lean muscle tissue is about 70% water. Adipose (fat) tissue is only about 20% water. That nearly fourfold difference is the single biggest reason body water percentages vary so much from person to person. Two people of the same height and age can have very different total body water simply because one carries more muscle and the other carries more fat. This is also why athletes and highly active people tend to sit at the upper end of the range, and why body water percentage is sometimes used as a rough proxy for body composition.
Where the Water Actually Sits
Not all of your body water sloshes around freely. About 60% of it is locked inside your cells, forming the fluid environment where most of your metabolism happens. The remaining 40% sits outside cells: in the fluid that bathes tissues, in blood plasma, and in smaller reservoirs like the fluid around your spinal cord and inside your eyes. Only about 20% of that extracellular portion is plasma, the liquid part of your blood. So while blood feels like the most “watery” part of you, it actually holds a relatively small share of your total water.
Water Content of Individual Organs
Some organs are surprisingly water-rich. According to the U.S. Geological Survey, the lungs top the list at about 83% water, which makes sense given that they need a thin film of moisture to exchange oxygen and carbon dioxide with every breath. The kidneys come in at 79%, reflecting the massive volume of fluid they filter each day. The brain and heart are both about 73% water. Even your skeleton, which feels like the driest part of your body, is roughly 31% water.
How Your Body Loses Water Daily
Your body sheds water constantly, even when you’re not exercising or visibly sweating. The minimum obligatory water loss for a sedentary adult in a comfortable climate breaks down roughly like this:
- Insensible losses (skin and breathing): about 800 mL per day. This is water that evaporates from your skin and leaves your lungs with every exhale, and you never notice it happening.
- Urine: at least 500 mL, the minimum your kidneys need to flush out waste products.
- Stool: about 200 mL.
- Minimal sweat: roughly 100 mL, even without exercise.
That adds up to about 1,600 mL (roughly 6.5 cups) lost daily before you factor in any physical activity, heat exposure, or illness. Exercise, hot weather, fever, or diarrhea can multiply these losses dramatically, which is why fluid needs vary so much from day to day.
How Much Water You Need to Replace
General guidelines suggest that healthy adults need somewhere between 11.5 cups (2.7 liters) and 15.5 cups (3.7 liters) of total fluid per day, with the higher end for men. That includes water from all sources: drinks, coffee, tea, and the water content of food. Fruits, vegetables, soups, and even meat contribute a meaningful amount. Most people who eat regular meals and drink when thirsty come reasonably close to meeting their needs without tracking ounces.
What Happens When You Lose Too Much
Your body is sensitive to even small water deficits. Losing just 1% to 2% of your body weight in fluid (about 1.5 to 3 pounds for an average adult) is enough to reduce endurance during exercise, impair concentration, and trigger thirst and fatigue. At a 2% loss, measurable declines in physical performance show up in nearly every study that tests it. By the time losses reach 5% of body weight, your capacity for physical work drops by about 30%, and your body becomes significantly worse at regulating its own temperature. The threshold for heat-related collapse shifts lower, from around 40°C (104°F) core temperature to closer to 39°C (102.2°F).
For context, a 160-pound person would need to lose about 8 pounds of water to hit that 5% mark. That level of dehydration is uncommon in everyday life but realistic during prolonged exercise in heat, especially if someone restricts fluids deliberately.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that displays body water percentage, it used a method called bioelectrical impedance analysis. These devices send a tiny electrical current through your body and measure how easily it passes. Water conducts electricity well, fat doesn’t, so the device estimates how much of your weight is water based on resistance. Consumer scales use a single frequency, while clinical-grade devices use multiple frequencies to separately estimate the water inside and outside your cells.
These devices are convenient and painless, but their accuracy varies depending on your hydration state at the moment, how recently you ate, and how well the device’s algorithm matches your body type. The gold standard for measuring total body water in research settings is isotope dilution, a technique where you drink a small amount of specially tagged water and then measure how diluted it becomes in your body. It’s precise but impractical for everyday use, which is why impedance scales remain the most common tool outside a lab.