The average adult body is about 60% water by weight. For a 70 kg (154 lb) person, that works out to roughly 42 liters, or about 11 gallons. But that number shifts considerably depending on your age, sex, and body composition.
How Body Water Varies by Age and Sex
Children start out with the highest water percentages. Boys and girls between ages 3 and 10 are roughly 62% water, with almost no difference between sexes. After puberty, the numbers diverge. Males at a healthy weight stay around 62-63% water through most of adulthood, then drop to about 57% after age 60. Females see a sharper decline: body water drops to about 55% in the teenage years, holds steady around 54% through middle age, then falls to roughly 50% after 60.
The reason for the sex difference comes down to body composition. Fat tissue contains only about 10% water, while lean tissue (muscle, organs) is 70-75% water. Because women typically carry a higher proportion of body fat than men, their overall water percentage is lower. The same principle explains the decline with aging: as people lose muscle mass and gain fat tissue over the decades, their total body water percentage drops.
Where All That Water Actually Lives
Your body water isn’t just sloshing around freely. It’s distributed across distinct compartments, each serving different functions. About two-thirds of your total body water sits inside your cells. For that average 70 kg person with 42 liters of water, roughly 28 liters are locked within cells, powering the chemical reactions that keep you alive.
The remaining third, about 14 liters, exists outside the cells. Most of that extracellular water (around 10.5 liters) fills the spaces between cells, bathing tissues and allowing nutrients and waste to move back and forth. The remaining 3.5 liters is your blood plasma, the liquid portion of blood that carries red blood cells, hormones, and dissolved nutrients through your circulatory system.
Water Content of Individual Organs
Not every organ holds the same amount of water. Your lungs are the most water-rich major organ at about 83%, which makes sense given the thin, moist membranes needed for gas exchange. The brain and heart are each about 73% water. Muscles and kidneys come in at 79%. Even your skin, which feels relatively dry on the surface, is 64% water. Bones are the driest tissue at 31%, though that’s still nearly a third water by weight.
Skeletal muscle deserves special attention because it makes up such a large share of body mass. At about 76% water, your muscles are essentially three-quarters liquid. This is one reason why muscular people tend to have a higher total body water percentage than people with more body fat. It also helps explain why hydration matters so much for physical performance: your muscles literally depend on water to maintain their structure and function.
How Your Body Holds and Releases Water
Your body stores water in some surprising ways. One notable example involves glycogen, the form of stored carbohydrate in your muscles and liver. Each gram of glycogen binds to at least 3 grams of water. This is why people on very low-carb diets often lose several pounds quickly in the first week. They’re burning through glycogen stores and releasing the water that was bound to it. It’s real weight loss, but it’s water, not fat.
On the output side, you lose water through routes you might not think about. Beyond urination and visible sweat, your body loses an estimated 600 to 800 milliliters of water every day through breathing, evaporation from skin, and stool. This “insensible” loss accounts for 30 to 50% of all daily water loss and happens whether or not you feel like you’re sweating.
How Your Body Keeps Water in Balance
Maintaining the right amount of water is so critical that your body has a finely tuned hormonal system dedicated to it. The key player is a hormone released by the brain that acts on the kidneys. When sensors in your brain detect that your blood is getting too concentrated (even a tiny shift triggers the response), this hormone signals your kidneys to reabsorb more water rather than sending it to the bladder. The result is smaller volumes of more concentrated urine, conserving water.
The same hormone responds to drops in blood volume or blood pressure. Pressure sensors in the heart and major arteries detect the change and relay the signal to the brain, which releases the hormone to both retain water through the kidneys and constrict blood vessels to maintain blood pressure. It’s a dual mechanism: save water and tighten the pipes.
This system works in reverse, too. When you drink a large amount of water, hormone levels drop, your kidneys stop reabsorbing as much, and you produce dilute urine in larger volumes. The speed of this adjustment is remarkable. Your body can shift from conservation mode to elimination mode within minutes.
How Much Water You Need Each Day
Total daily fluid needs for healthy adults fall between about 11.5 cups (2.7 liters) and 15.5 cups (3.7 liters), with the higher end typically applying to men. That total includes water from all sources: plain water, other beverages, and the water content of food. Fruits, vegetables, soups, and even meats contribute meaningful amounts of water to your daily intake, so you don’t need to drink all of it from a glass.
Your actual needs shift with activity level, climate, altitude, and health status. Exercise, hot weather, and illness with fever or vomiting all increase losses and push your requirements higher. Thirst is a generally reliable guide for most healthy adults, though it becomes less sensitive with age, which is one reason older adults are more vulnerable to dehydration.
Why the Percentage Matters
The fact that your body is mostly water isn’t just a fun statistic. Water is the solvent for virtually every biochemical reaction in your cells, the medium that carries oxygen and nutrients to tissues, the fluid that cushions your brain and spinal cord, and the coolant that regulates your temperature through sweat. Losing even a small percentage of body water, around 2% of body weight, can impair physical endurance and cognitive function. Losses beyond 5% become medically significant, and beyond 10% can be life-threatening.
Your body’s water content also has practical implications for how medications are distributed, how alcohol affects you, and how quickly you recover from illness. People with lower body water percentages (older adults, people with higher body fat) will reach higher blood concentrations of water-soluble substances from the same dose, which is one reason medication dosing sometimes varies by age, sex, and weight.