Water makes up about 50% to 60% of your total body weight, depending on your age, sex, and body composition. For an average adult man, that figure is roughly 59%, while for an adult woman it’s closer to 50%. In real terms, a 180-pound man carries around 106 pounds of water, and a 140-pound woman carries roughly 70 pounds.
How Water Percentage Changes With Age and Sex
You’re born with far more water in your body than you’ll carry as an adult. Newborns average about 74% water, with some as high as 84%. By six months, that drops to around 60%, where it holds relatively steady through childhood.
In adulthood, the gap between men and women widens. Men aged 19 to 50 average 59% body water, with a broad range of 43% to 73%. Women in the same age group average 50%, ranging from 41% to 60%. The primary reason for this difference is body fat: women naturally carry more fat tissue, and fat holds far less water than muscle. Lean tissue is about 73% water, while fat tissue can be as low as 17%.
After age 50, water percentage declines further. Men average about 56%, women about 47%. This happens partly because people tend to lose muscle mass and gain fat as they age, and partly because the body’s ability to conserve water becomes less efficient over time.
Where the Water Actually Sits
Not all of your body’s water is sloshing around in one place. About 62% of it sits inside your cells, where it’s essential for chemical reactions that keep you alive. The remaining 38% is outside your cells: in your blood, in the fluid between tissues, and in places like your spinal canal and the space around your joints.
Different organs hold dramatically different amounts of water. Your lungs are the most water-rich organ at about 83%, which makes sense given how much moisture is involved in gas exchange. Your brain and heart are both roughly 73% water. Muscles and kidneys come in at 79%, skin at 64%, and even your bones, which feel completely solid, are 31% water. Blood plasma, the liquid portion of your blood that carries cells and nutrients, is 90% water.
Why Body Composition Matters More Than Weight
Two people who weigh the same can have very different water percentages. The biggest factor is how much of your body is muscle versus fat. Lean tissue is consistently about 73% water. Fat tissue is far more variable, anywhere from 17% to 84% depending on the type and location, but it averages much lower than muscle. This is why a muscular athlete might test at 65% body water while someone of the same weight with more body fat might be closer to 45%.
This also explains the sex difference. It’s not that women’s cells contain less water. It’s that women’s bodies typically carry a higher proportion of fat tissue, which dilutes the overall percentage. A woman with an unusually high amount of lean muscle mass may have a body water percentage that overlaps with the male average.
How Your Body Keeps Water in Balance
Your body regulates water levels through a tightly controlled feedback loop centered in the brain. Specialized sensors in a brain region called the hypothalamus constantly monitor the concentration of salts and other dissolved particles in your blood. When that concentration rises, meaning you’re getting even slightly dehydrated, these sensors trigger the release of a hormone that travels to your kidneys.
This hormone makes the kidneys more permeable to water by opening tiny channels in the kidney walls. Water that would normally be lost as urine gets reabsorbed back into the bloodstream instead, so your urine becomes more concentrated and your blood volume stays stable. Once the salt concentration drops back to normal, the hormone signal tapers off, the channels close, and your kidneys resume producing more dilute urine. The whole system runs on a continuous feedback loop, adjusting in real time throughout the day.
This is why your urine color shifts from pale yellow to dark amber depending on how hydrated you are. Dark urine means your kidneys are working hard to conserve water. Pale urine means you have plenty to spare.
What Happens When Water Drops Too Low
Dehydration is measured as the percentage of body weight lost through fluid. In children, losing up to 3% of body weight in fluid counts as mild dehydration. At 6%, it’s moderate. At 9% or above, it becomes severe and potentially dangerous. Infants are more vulnerable and can tolerate slightly different thresholds, with up to 5% considered mild and 10% to 15% considered severe.
In practical terms, mild dehydration in an adult might mean losing about 2 to 3 pounds of fluid for a 150-pound person. At this level you’ll notice thirst, darker urine, a dry mouth, and possibly a headache or fatigue. Moderate dehydration adds dizziness, rapid heartbeat, and reduced urine output. Severe dehydration is a medical emergency that can cause confusion, fainting, and organ damage.
Most healthy adults replace lost water naturally through thirst. But the thirst response weakens with age, which is one reason older adults are more prone to dehydration even in mild conditions.
How Much Water You Need Each Day
The National Academies set the recommended total water intake at 3.7 liters per day for men and 2.7 liters per day for women. These numbers include all sources of water: drinking water, other beverages, and the water naturally present in food. About 80% of most people’s water intake comes from drinks, with the remaining 20% from food.
That breaks down to roughly 13 cups of beverages per day for men and 9 cups for women. “Beverages” here means anything you drink: water, coffee, tea, juice, milk. Food-heavy water sources include fruits, vegetables, soups, and yogurt. A cucumber is about 95% water, and even bread contains some.
These guidelines stay the same across adult age groups, from 19 through 70 and beyond. But individual needs vary based on climate, physical activity, body size, and health conditions. If you’re sweating heavily during exercise or spending time in hot weather, your needs can increase substantially.
Measuring Your Body Water Percentage
The most accessible method for estimating body water is bioelectrical impedance analysis, which many smart scales and body composition devices use. The technology works by sending a small electrical current through your body. Water conducts electricity well, so the device measures how easily the current passes through to estimate how much of your weight is water, fat, and lean tissue.
These devices are convenient but imperfect. Their readings can shift by up to 1% based simply on whether you recently drank something, and fluctuations of 0.7% can occur even without any change in hydration. They’re useful for tracking trends over time but shouldn’t be treated as precise measurements on any single reading. For the most accurate results, use the device at the same time of day, under similar conditions, and avoid eating or drinking for at least a couple of hours beforehand.