The human body is roughly 60% water in adult men and about 50% in adult women. That number isn’t fixed, though. It shifts across your lifetime, varies between individuals, and depends heavily on your body composition. Understanding where all that water actually sits in your body, and why the percentage changes, makes the simple answer a lot more interesting.
How Body Water Changes With Age
At birth, a baby’s body is about 80% water. That percentage drops steadily through childhood and adolescence as body fat increases and tissues mature. By adulthood, men average around 60% and women around 50%, a gap largely explained by differences in body fat (more on that below).
The decline doesn’t stop there. After age 60, both men and women lose water content as muscle mass naturally decreases. Men in their sixties and beyond average about 57%, while women drop to around 50%. This age-related decline is one reason older adults are more vulnerable to dehydration: they have a smaller water reserve to begin with, and their thirst signals often become less reliable.
Why Men and Women Differ
The roughly 10-percentage-point gap between men and women comes down to tissue composition. Muscle is a water-rich tissue. Fat is not. Lean, fat-free mass is about 80% water by weight, while adipose (fat) tissue contains only about 14% water. That means fat-free tissue holds roughly six times more water per unit of weight than fat does.
Because women on average carry a higher proportion of body fat than men, their overall water percentage is lower. The same principle applies regardless of sex: a highly muscular person will have a higher body water percentage than someone of the same weight with more body fat. Athletes, for instance, often test above the 60% average, while someone with a higher body fat percentage may fall below 50%.
Where Water Lives in Your Body
Water isn’t spread evenly across your organs and tissues. Some parts are surprisingly waterlogged. Your lungs are about 83% water, making them the most water-dense major organ. The brain and heart are each about 73% water. Even your bones, which feel solid and dry, are roughly 31% water.
At the cellular level, about two-thirds of your body’s water sits inside cells (intracellular fluid), where it’s essential for chemical reactions and energy production. The remaining third is extracellular: it circulates in your blood plasma, fills the spaces between cells, and cushions joints and organs.
What All That Water Actually Does
Water isn’t just filling space. It serves as the medium for nearly every biological process in your body.
- Temperature regulation: When your body heats up, you sweat. As that sweat evaporates from your skin, it pulls heat away and cools you down. Sweat production can range from about 0.3 liters per hour at rest to 2.0 liters per hour during intense exercise in the heat.
- Waste removal: Your kidneys filter waste products from the bloodstream and flush them out through urine. A minimum volume of urine is required to clear the body’s daily waste load, with the kidneys capable of producing up to about 1 liter per hour at maximum output.
- Nutrient transport: Water in blood plasma carries oxygen, glucose, and other nutrients to cells throughout the body, then carries waste products back to the kidneys and lungs for disposal.
- Structural support: Water cushions the brain and spinal cord, lubricates joints, and helps maintain the shape and volume of individual cells.
When you lose water through sweat without replacing it, your blood volume drops and its concentration of dissolved particles rises. This makes it harder for your body to continue sweating effectively, and your core temperature can climb. That’s the physiological chain reaction behind heat exhaustion.
How Much Water You Lose Each Day
Your body cycles through a surprising amount of water in 24 hours. Research tracking daily water losses in young adults found that urine accounts for about 76.5% of total daily losses, or roughly 1,295 mL. Evaporation through the skin adds another 10.3% (about 172 mL), breathing accounts for 9.5% (about 149 mL), and a small amount leaves through the digestive tract at about 3.6% (64 mL).
Under normal conditions, adults need roughly 2.5 to just over 3 liters of total water per day from all sources, including food. In extreme heat with heavy physical activity, that requirement can reach 6 liters per day. The old “eight glasses a day” rule is a reasonable starting point, but your actual needs depend on your size, activity level, climate, and diet. Fruits, vegetables, soups, and other foods contribute a significant portion of daily water intake.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that estimates your body fat and water percentages, it was using a technique called bioelectrical impedance analysis. This method sends a tiny electrical current through your body and measures resistance. Water conducts electricity well, so the current flows more easily through well-hydrated lean tissue than through fat.
Multi-frequency versions of this technology, which use several different electrical frequencies, tend to give more accurate readings than single-frequency devices. Clinical settings also use these devices, along with more precise (and expensive) methods involving isotope dilution, where you drink a small amount of labeled water and measure how it distributes. For most people, though, the quick impedance method provides a reasonable estimate.
Factors That Shift Your Percentage
Several things can push your body water percentage up or down on any given day. Dehydration from exercise, illness, or simply not drinking enough will temporarily lower it. Hormonal fluctuations during the menstrual cycle cause fluid retention that can shift readings by a few percentage points. Medications like diuretics actively reduce body water. Even the time of day matters: you’re typically most dehydrated in the morning after hours without fluid intake.
Over longer periods, changes in body composition have the biggest impact. Gaining muscle raises your water percentage; gaining fat lowers it. This is why fitness-focused body composition scales track water percentage alongside fat: the two numbers move in opposite directions, and together they give a more complete picture of what’s changing inside your body.