The average adult human body is about 60% water by weight for men and 52 to 55% for women. That means a 180-pound man carries roughly 108 pounds of water, distributed across every organ, tissue, and cell. The exact percentage varies based on age, sex, and body composition, so two people standing side by side can have meaningfully different water content.
Why the Percentage Varies From Person to Person
The single biggest factor is body fat. Fat tissue contains only about 14% water, while fat-free mass (muscle, organs, bone) is roughly 80% water. That’s a sixfold difference per unit of weight. Because women typically carry a higher percentage of body fat than men, their total body water percentage runs lower. For the same reason, a lean, muscular person of either sex will have a higher water percentage than someone with more body fat at the same weight.
Age is the other major driver. Newborns are roughly 74% water, with a range as wide as 64 to 84%. By six months to a year, that settles to around 60%. Adults over 51 see a further decline: men in that age group average about 56%, while women average around 47%. The drop happens partly because older adults tend to lose muscle mass and gain fat tissue, and partly because the body’s ability to regulate fluid balance becomes less efficient over time.
Where the Water Actually Sits
Your body’s water isn’t sloshing around freely. About two-thirds of it sits inside your cells, forming the medium where most chemical reactions take place. The remaining third is outside cells: in your blood plasma, in the fluid surrounding tissues, and in smaller reservoirs like cerebrospinal fluid and the fluid in your eyes. This distribution is sometimes called the 60-40-20 rule. If total body water is 60% of your weight, roughly 40% is inside cells and 20% is outside them.
That balance matters. Your body actively manages the concentration of dissolved salts on each side of cell membranes to keep water flowing where it’s needed. When you’re dehydrated or when electrolytes are off, the shift between these compartments is what produces symptoms like swelling, low blood pressure, or muscle cramps.
Water Content of Individual Organs
Not every part of your body holds water equally. According to the U.S. Geological Survey, the lungs are the most water-rich major organ at about 83%. The brain and heart each come in at around 73%. Even bones, which feel solid and dry, are about 31% water. Muscle tissue falls in between, with a water content significantly higher than fat but lower than the lungs.
This explains why dehydration hits your brain early. Losing just 2% of your body water is enough to impair concentration, slow reaction times, affect short-term memory, and shift your mood in a negative direction. For a 150-pound person, 2% is only about 3 pounds of water, an amount you can lose through sweat during a single hard workout on a hot day.
What All That Water Does
Water is the solvent your body uses for nearly every biological process. Nutrients from food dissolve in it so they can be absorbed through the intestinal wall and carried to cells via the bloodstream. Waste products dissolve in it so your kidneys can filter them out. The chemical reactions that convert food into energy happen in water inside your cells.
Temperature regulation depends on water, too. When you overheat, your body pushes water to the skin’s surface as sweat. As that sweat evaporates, it pulls heat away from your body. This system works remarkably well, but only if you have enough fluid to spare. It’s also why dehydration and heat illness tend to arrive together.
Water also provides structural support. The fluid in your joints acts as a cushion and lubricant. The water-rich discs between your vertebrae absorb shock. Even your eyes maintain their shape partly through internal fluid pressure.
How Much Fluid You Need Daily
General guidance from the Mayo Clinic puts adequate daily fluid intake at about 15.5 cups (3.7 liters) for men and 11.5 cups (2.7 liters) for women. That total includes all fluids: water, coffee, tea, milk, and the water contained in food. Fruits, vegetables, soups, and even cooked grains contribute a meaningful share. Most people get roughly 20% of their daily water from food alone.
These numbers are averages for healthy adults in temperate climates with moderate activity levels. You’ll need more if you exercise intensely, spend time in heat or high altitude, are pregnant or breastfeeding, or are running a fever. Thirst is a reasonably good guide for most healthy people, though it becomes less reliable in older adults, who may not feel thirsty even when their fluid levels are low.
The color of your urine is a practical check. Pale yellow generally signals adequate hydration. Dark yellow or amber suggests you need more fluid. Clear and colorless for extended periods may mean you’re drinking more than necessary, though that’s rarely harmful for someone with normal kidney function.