The average adult human body is roughly 60% water by weight, though that number shifts depending on your age, sex, and body composition. For a 154-pound (70 kg) person, that works out to about 42 liters, or roughly 11 gallons, of water distributed throughout your tissues, organs, and bloodstream.
How Body Water Varies by Person
That 60% figure is a useful average, but your actual percentage could be higher or lower. Men typically carry more water than women because they tend to have more muscle mass, and muscle holds significantly more water than fat. Fat-free tissue is about 73% water, while adipose (fat) tissue contains far less. This is why two people at the same weight can have very different total body water levels depending on how much of their weight comes from muscle versus fat.
Age plays an even bigger role. Newborns are roughly 75% water. That percentage gradually decreases through childhood and adulthood, then drops further in older age. The reason is straightforward: as you get older, you naturally lose muscle mass, and less muscle means less water storage capacity. According to UCLA Health, up to 40% of adults older than 65 experience chronic dehydration, partly because their bodies simply can’t hold as much water as they once could. This makes older adults more vulnerable to fatigue, confusion, and serious complications from even mild fluid losses.
Where All That Water Actually Sits
Your body water isn’t sloshing around in one big pool. It’s divided into two main compartments in roughly a 2:1 ratio. About two-thirds of your total body water is intracellular, meaning it’s inside the trillions of cells throughout your tissues. The remaining third is extracellular, sitting outside your cells.
That extracellular portion breaks down further. About 75% of it is interstitial fluid, the liquid that fills the spaces between cells in your tissues, accounting for roughly 15% of your body mass. The other 25% is plasma, the liquid portion of your blood, making up about 5% of body mass. A small fraction (2 to 3%) exists in specialized compartments like the fluid surrounding your brain and spinal cord, your lymphatic system, and your eyes.
Some Organs Hold Far More Water Than Others
Not every part of your body is equally waterlogged. Your lungs are the most water-rich major organ at about 83%. The brain and heart come in at around 73%, and muscles and kidneys are close behind at 79%. Even your skin, which feels relatively dry to the touch, is 64% water. Bones are the driest major tissue, but they still contain a meaningful amount of fluid.
These differences matter. Organs that are metabolically active, meaning they burn a lot of energy and exchange a lot of molecules, need more water to function. Your brain, for instance, relies on all that water to cushion itself, deliver nutrients, and remove waste products. When you’re dehydrated, the organs with the highest water demands tend to feel the effects first, which is why concentration, mood, and physical performance can decline well before you feel truly thirsty.
What Your Body Uses All That Water For
Water isn’t just filling space. It’s the medium your body uses for nearly every critical process. The proteins and carbohydrates you eat are broken down and carried to cells through the bloodstream, which is mostly water. Your joints stay cushioned because water-rich fluid acts as a lubricant between bones. When you overheat, water leaves your body as sweat and evaporates from your skin, pulling heat away and cooling you down. Even the simple act of breathing releases water vapor, which helps regulate temperature from the inside.
At a chemical level, water is the solvent that makes cellular reactions possible. Nutrients dissolve in it, waste products dissolve in it, and the electrical signals your nerves use to communicate depend on charged particles (electrolytes) dissolved in water. Without adequate fluid, these processes slow down or malfunction, which is why dehydration affects so many systems at once.
How Your Body Keeps Water in Balance
You lose about 2.5 liters (roughly 10.5 cups) of water every day through urine, sweat, breathing, and bowel movements. Urine accounts for the largest share, averaging about 1.5 liters daily. The rest exits through your skin, lungs, and digestive tract. To stay in balance, you need to take in roughly the same amount you lose.
Your body manages this balance through a surprisingly precise feedback system. Specialized sensors in your brain constantly monitor the concentration of your blood. When you lose water and your blood becomes slightly more concentrated, these sensors trigger the release of a hormone that tells your kidneys to reabsorb more water instead of sending it to your bladder. The result is smaller volumes of more concentrated urine. When you’re well-hydrated, the opposite happens: the hormone signal drops, your kidneys let more water pass through, and you produce larger volumes of dilute urine. This is why urine color is a rough but useful indicator of hydration. Pale yellow generally means adequate fluid; dark yellow suggests your body is conserving water.
The same sensors also trigger thirst, though this signal isn’t always reliable. In older adults especially, thirst sensation becomes blunted, which is one reason chronic dehydration is so common in that age group.
How Much Water You Need Each Day
General guidelines suggest 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 typically applying to men. That number includes all fluids, not just plain water. About 20% of your daily water intake comes from food, particularly fruits, vegetables, soups, and other moisture-rich items. The remaining 80% comes from beverages of all kinds.
Your actual needs depend on how much water you’re losing. Exercise, hot weather, illness involving fever or vomiting, and high altitude all increase water loss and push your requirements higher. Pregnancy and breastfeeding also increase fluid demands. Rather than fixating on a specific cup count, paying attention to urine color and thirst gives most people a practical way to gauge whether they’re drinking enough.
Why Body Composition Matters More Than Weight
If you’ve ever wondered why hydration advice isn’t one-size-fits-all, body composition is the main reason. A muscular 180-pound person holds substantially more water than a 180-pound person with a higher body fat percentage, because lean tissue is roughly 73% water while fat tissue stores much less. This difference also explains why women, who on average carry a higher proportion of body fat than men, tend to have a lower total body water percentage.
This has practical implications beyond trivia. People with less lean mass have a smaller water reserve to draw from during exercise, illness, or heat exposure. They reach a state of meaningful dehydration faster, and they feel the cognitive and physical effects sooner. Building or maintaining muscle mass, particularly as you age, isn’t just about strength. It’s one of the most effective ways to preserve your body’s ability to stay hydrated.