The average adult human body is 50 to 65 percent water by weight. For a 154-pound (70 kg) person, that works out to roughly 10 to 11 gallons of water. The exact percentage depends on your age, sex, and how much body fat you carry, which is why a single number never tells the whole story.
What Determines Your Body’s Water Content
Men typically fall in the 60 to 65 percent range, while women tend to be closer to 50 to 60 percent. The difference comes down to body composition. Fat tissue holds only about 14 percent water, while fat-free tissue (muscle, organs, bone) is roughly 80 percent water. That means fat-free tissue contains about six times more water per pound than fat. Since women on average carry a higher proportion of body fat, their overall water percentage is lower.
Age matters too. Infants are about 70 percent water, which is part of why dehydration in babies can escalate quickly. From there, the number gradually drops. Between age 20 and 70, total body water declines by about 3.7 percent as muscle mass decreases and body fat tends to increase. Older adults may fall below 50 percent.
The commonly repeated claim that “the human body is 70 percent water” is overstated for most adults. That figure is closer to what you’d see in a newborn. For a typical adult, 55 to 60 percent is a more realistic number.
Water Content of Individual Organs
Not all tissues hold the same amount of water. Your lungs are the most water-rich major organ at about 83 percent water by weight, which makes sense given the thin, moist membranes needed for gas exchange. The brain and heart are both about 73 percent water. Muscles and kidneys come in at roughly 79 percent. Even bones, which feel completely solid, are around 31 percent water.
Skin, your largest organ, is about 64 percent water. This is one reason dehydration shows up visibly: when your skin’s water content drops, it loses elasticity and can look dull or feel tight.
Where All That Water Sits
Your body divides its water into two main compartments. About 60 percent of total body water is inside your cells, called intracellular fluid. This is the water that fills each of the roughly 37 trillion cells in your body and provides the environment for all the chemical reactions that keep you alive.
The remaining 40 percent sits outside cells as extracellular fluid. This includes blood plasma, the fluid between cells (interstitial fluid), and smaller volumes in places like the spinal cord, joints, and eyes. Your body carefully regulates the balance between these compartments. When you’re dehydrated, water shifts between them in predictable ways, which is partly how your kidneys and hormones maintain blood pressure and cell function.
How Your Body Gains and Loses Water Daily
You cycle through a surprising volume of water every day. On the loss side, urine accounts for the largest share. Depending on your fluid intake and kidney function, you can excrete anywhere from about half a liter to over 10 liters per day, though 1 to 2 liters is typical. You also lose close to a liter daily through skin evaporation and breathing combined, even without visible sweating. A small amount leaves through your digestive tract, though severe diarrhea or vomiting can push digestive water loss above a gallon a day.
To stay in balance, you replace that water through drinking, eating (many foods are 80 to 90 percent water), and a small amount generated internally when your body breaks down nutrients for energy. The total turnover for a sedentary adult in a moderate climate is roughly 2.5 liters per day, but exercise, heat, and illness can push it much higher.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that estimates body water, it was using a technique called bioelectrical impedance analysis. The scale sends a tiny, painless electrical current through your body. Water conducts electricity well, so the device measures resistance and uses that, along with your height and weight, to estimate total body water. These consumer devices give a reasonable ballpark, but they can be off by 2 to 3 liters depending on how hydrated you are at the moment, whether you’ve just eaten, or how much you’ve been sweating.
In clinical and research settings, the gold standard is a dilution method: you drink a small dose of water labeled with a traceable marker (typically a harmless form of hydrogen called deuterium), wait for it to distribute throughout your body, then provide a blood or saliva sample. By measuring how diluted the marker becomes, researchers can calculate your total water volume with high accuracy. When bioelectrical impedance equations are calibrated against this method, they can predict total body water with a correlation above 0.95, which is why the technique has become a practical alternative outside the lab.
Why Body Water Percentage Matters
Your body’s water percentage isn’t just trivia. It’s a rough proxy for body composition and metabolic health. A higher water percentage generally reflects more lean tissue relative to fat. Athletes and people with more muscle mass tend to sit at the higher end of the range, while a lower percentage can indicate higher body fat or, in older adults, the gradual loss of muscle that comes with aging.
Water also drives nearly every major function in your body. It acts as a solvent for nutrients and waste products, cushions joints and the spinal cord, helps regulate body temperature through sweat, and provides the medium in which every cellular reaction takes place. Even a 2 percent drop in body water, roughly a liter for most adults, is enough to impair concentration, increase fatigue, and reduce physical performance. By the time you feel thirsty, you’re typically already mildly dehydrated.