Water makes up roughly 60% of an adult man’s body weight and 50% to 55% of an adult woman’s. For a 154-pound man, that works out to about 42 liters, or just over 10.5 gallons, of water carried around at all times. The exact percentage depends on your age, sex, and body composition.
How Body Water Changes With Age and Sex
Children carry proportionally more water than adults. Boys and girls aged 3 to 10 are about 62% water by weight. During puberty, the numbers start to diverge between sexes, largely because of differences in body fat. Women tend to carry more fat tissue than men, and fat holds far less water than muscle does. By adulthood, men average around 60% and women around 50% to 55%.
These percentages hold fairly steady through middle age, then decline. After age 60, men drop to roughly 57% water and women to about 50%. This shift happens because older adults tend to lose lean muscle mass and gain fat tissue. It’s also one reason older adults are more vulnerable to dehydration: there’s simply less of a water reserve to draw from.
Why Body Fat Makes Such a Big Difference
Lean tissue, which includes your muscles and organs, is about 73% water. Fat tissue is far drier, and its water content is highly variable, ranging anywhere from 17% to 84% depending on the type and location of the fat. On average, though, fat holds substantially less water than muscle. This is the single biggest reason two people of the same weight can have very different total body water levels. A muscular person carries more water than someone of the same weight with a higher body fat percentage.
Where All That Water Actually Sits
Not all of your body’s water is in one place. About 60% of it sits inside your cells, forming the fluid where most of your body’s chemical reactions happen. For a 154-pound man, that’s roughly 28 liters tucked inside cells throughout the body. The remaining 40% is outside the cells: about 10.5 liters fills the spaces between cells in your tissues, and another 3.5 liters circulates in your blood. That blood portion, small as it sounds, is critical for delivering oxygen and nutrients and carrying away waste.
Water Content of Individual Organs
Some parts of your body are far more water-dense than others. Your muscles and kidneys are roughly 79% water, making them among the most water-rich tissues. Your skin comes in at about 64%. Even your bones, which feel completely solid, are 31% water. These numbers help explain why dehydration affects so many systems at once. When your water levels drop, every organ from your kidneys to your skeleton is operating with less of the substance it depends on.
How You Lose Water Every Day
Your body is constantly cycling through its water supply. On a typical day without heavy exercise, you lose water through five main routes:
- Urine: 1,000 to 1,500 mL, or roughly 1 to 1.5 quarts. This is your largest source of daily water loss.
- Sweat: 500 to 700 mL under normal conditions, more in heat or during exercise.
- Insensible perspiration: 250 to 350 mL of water seeps through your skin even when you’re not visibly sweating.
- Breathing: 250 to 350 mL escapes as water vapor every time you exhale.
- Stool: 100 to 200 mL leaves through your digestive tract.
Add those up and you’re losing somewhere around 2.1 to 3.1 liters of water per day just through normal bodily functions. That’s before factoring in exercise, hot weather, illness, or anything else that increases fluid loss. Your body replaces this through the water and other fluids you drink, the moisture in food, and a small amount generated internally when your cells break down nutrients for energy.
What Happens When Water Levels Drop
Because water is involved in virtually every process in your body, even small deficits have noticeable effects. Losing just 1% to 2% of your body water, an amount that can happen during a busy day when you forget to drink, typically causes thirst, fatigue, and reduced concentration. Physical performance drops, reaction times slow, and you may notice a headache or mild dizziness. By the time you feel genuinely thirsty, you’re often already mildly dehydrated.
More severe losses, above 3% to 5%, can impair your body’s ability to regulate temperature, reduce blood volume enough to strain your heart, and cause confusion. This is why water balance matters so much in older adults, young children, and anyone exercising in the heat. Their margin for error is smaller.
What Keeps Your Water Level Stable
Your body tightly controls its water balance through a feedback loop centered on your kidneys. When you’re well hydrated, your kidneys produce more dilute urine to shed the excess. When you’re running low, they concentrate your urine to conserve water, and your brain triggers the sensation of thirst to push you toward drinking. Hormones fine-tune this process minute to minute, adjusting how much water your kidneys retain based on signals from sensors that detect changes in blood volume and concentration.
This system is remarkably precise, but it has limits. It works best when you give it something to work with by drinking fluids consistently throughout the day rather than trying to catch up all at once. Food contributes too: fruits, vegetables, soups, and other water-rich foods can supply a meaningful portion of your daily intake, sometimes 20% or more depending on your diet.