Your body is roughly 50 to 60 percent water by weight. For a 154-pound (70 kg) person, that translates to about 38 to 42 liters, or roughly 10 to 11 gallons sloshing around inside you at any given moment. The exact percentage depends on your age, sex, and how much body fat you carry.
Your Water Percentage by Age and Sex
Babies are the most water-dense humans. From birth to six months, an infant’s body averages about 74 percent water, with a range of 64 to 84 percent. That proportion drops steadily through childhood and into adulthood as body composition shifts.
For adults between 19 and 50, men average about 59 percent water (range: 43 to 73 percent), while women average about 50 percent (range: 41 to 60 percent). The gap between men and women comes down to body fat. Fat tissue holds far less water than lean muscle. Since women typically carry a higher percentage of body fat, their total body water runs lower. Lean muscle tissue is about 73 percent water, while fat tissue is highly variable and can range from as low as 17 percent to over 80 percent water depending on the type and location.
After age 50, those numbers dip a bit further. Men average around 56 percent and women around 47 percent. The decline reflects the gradual loss of muscle mass that comes with aging, replaced in part by fat tissue that stores less water.
Where the Water Actually Sits
Not all of your body water is in one place. About two-thirds of it sits inside your cells, forming the fluid environment where most of your body’s chemistry happens. The remaining one-third is outside your cells: in your blood, in the fluid between tissues, and in places like your spinal cord and eyes. This is sometimes called the 60-40-20 rule. If 60 percent of your body weight is water, 40 percent is the water inside cells and 20 percent is the water outside them.
Your blood plasma, the liquid portion of your blood, is one of the most visible extracellular fluids. But the largest share of extracellular water actually lives in the spaces between your cells, bathing tissues and allowing nutrients and waste to move back and forth.
How Much Water Each Organ Holds
Some organs are far more water-rich than you might expect. According to the U.S. Geological Survey, your lungs are the most water-dense major organ at about 83 percent water. Your kidneys come in at 79 percent. Your brain and heart are each about 73 percent water. Even your bones, which feel completely solid, contain a meaningful amount of water.
This is why dehydration can affect so many systems at once. When your brain is nearly three-quarters water, even a small fluid deficit can influence concentration, mood, and energy levels. When your kidneys are almost 80 percent water, they need steady fluid intake to filter waste efficiently.
What All That Water Does
Water isn’t just filling space. It serves as the solvent for nearly every chemical reaction in your body, from digesting food to building new cells. It cushions your brain and spinal cord, lubricates your joints, and helps transport nutrients through your bloodstream to every tissue.
One of its most critical jobs is temperature regulation. When you exercise or sit in the heat, your body pushes water to the surface of your skin as sweat. As that sweat evaporates, it pulls heat energy away from your body and cools you down. The rate of cooling depends on humidity: dry air lets sweat evaporate quickly, while humid air slows the process, which is why hot, muggy days feel so much more oppressive. Without adequate water to produce sweat, your core temperature can rise dangerously fast.
Why Body Composition Matters More Than Weight
Two people who weigh exactly the same can have very different amounts of total body water. A muscular 180-pound person might be 65 percent water, while a 180-pound person with a higher body fat percentage might be closer to 45 percent. This is because lean tissue is consistently about 73 percent water, while fat tissue holds much less. So your fitness level and body composition are stronger predictors of total body water than your weight alone.
This also explains why athletes can lose several pounds during a single workout. That weight loss is almost entirely water lost through sweat and respiration, not fat. A 2 percent drop in body water, which works out to roughly 3 pounds for a 150-pound person, is enough to noticeably impair physical performance and mental sharpness.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that estimates body fat and water percentage, you’ve used a technology called bioelectrical impedance analysis. It sends a tiny, imperceptible electrical current through your body, typically entering through electrodes on your feet or hands. Because water conducts electricity well and fat does not, the device measures how much resistance your tissues put up and uses that to estimate total body water.
These consumer devices give a reasonable ballpark but aren’t highly precise. They can be thrown off by how hydrated you are at that moment, whether you’ve just eaten, or even whether your skin is damp. The gold-standard method in research settings involves drinking a small dose of water labeled with a harmless hydrogen isotope, then measuring how it distributes through the body via a urine sample taken a few hours later. This dilution technique is accurate but impractical for everyday use.
Keeping Your Body Water in Balance
Your body is constantly losing water through urine, sweat, breathing, and digestion, and it relies on you to replace it. General guidelines suggest that healthy adults need roughly 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day from all sources, including the water in food. Fruits, vegetables, soups, and other moisture-rich foods contribute a meaningful share, often around 20 percent of your daily intake.
Your actual needs shift based on activity level, climate, body size, and health status. Thirst is a decent guide for most healthy people, though it lags slightly behind actual need. By the time you feel thirsty, you may already be mildly dehydrated. Urine color is a more reliable real-time indicator: pale yellow generally signals good hydration, while dark amber suggests you need more fluid.