Water makes up about 50% to 65% of your body weight if you’re an average adult. The exact number depends on your age, sex, and body composition, and it shifts throughout your lifetime. A newborn is roughly 70% to 80% water, while an older adult may fall below 50%.
How Water Percentage Changes With Age
Infants carry the highest proportion of water of any age group, typically 70% to 80% of their body weight. This supports the rapid cell division and high metabolic rate of early development. As children grow and gain more fat and bone mass relative to their size, that percentage gradually drops.
By adulthood, most people settle into the 50% to 65% range. The decline continues into older age as lean tissue naturally decreases and the body’s ability to conserve water becomes less efficient. Elderly adults often have the lowest body water percentages, which is one reason they’re more vulnerable to dehydration, even from mild illness or a hot day.
Why Men and Women Differ
The average adult male is about 60% water, while the average adult woman is closer to 55%. The gap comes down to body fat. Fat tissue holds considerably less water than lean tissue like muscle, and women naturally carry a higher percentage of body fat than men. For the same reason, a very lean person of either sex will have a higher water percentage than someone who is overweight, regardless of gender.
Where the Water Actually Sits
Not all of your body’s water is sloshing around in one pool. About 62% of your total body water is stored inside your cells, where it’s essential for chemical reactions, energy production, and maintaining cell structure. The remaining 38% sits outside cells: in blood plasma, in the fluid between tissues, and in smaller reservoirs like spinal fluid and the moisture lining your digestive tract.
Blood plasma itself is about 92% water. That high water content is what allows plasma to carry nutrients, hormones, and waste products efficiently through your circulatory system.
Water Content of Individual Organs
Some organs are far more water-dense than you might expect. According to data compiled by the U.S. Geological Survey, the lungs top the list at about 83% water, which makes sense given their need to stay moist for gas exchange. The brain and heart are each around 73% water. Muscles and kidneys come in at 79%, and skin at 64%. Even bones, which feel completely solid, are about 31% water.
These numbers help explain why dehydration can produce such wide-ranging symptoms. When your total body water drops, organs that depend heavily on water, like your brain, are among the first to signal trouble through headaches, difficulty concentrating, and fatigue.
How Much Water You Cycle Through Daily
Your body doesn’t just store water passively. It runs through a surprisingly large volume every day. Research tracking water turnover in American adults found that men take in an average of 3.6 liters of water per day from all sources (beverages, food moisture, and water generated by metabolism), while women average about 3.0 liters. On the output side, urine alone accounts for roughly 2.2 liters per day for both sexes, with the rest lost through breathing, sweat, and digestion.
This constant cycling means your body’s water supply is being fully refreshed on a regular basis. It also means that even modest changes in intake or output, like skipping fluids on a busy day or sweating heavily during exercise, can shift your hydration status quickly.
What Dehydration Looks Like by Percentage
Clinicians measure dehydration as the percentage of body weight lost through water. The thresholds differ slightly for infants and older children, but the general pattern is the same.
Mild dehydration, up to about 3% to 5% body weight loss, usually looks like thirst, slightly dry lips, and a normal pulse. You’re still alert and active. Moderate dehydration (roughly 6% to 10% in infants, around 6% in older children and adults) brings irritability, a faster heart rate, noticeably dry mouth, and reduced tears. If you pinch the skin, it may take a moment to spring back instead of snapping flat immediately.
Severe dehydration, at 9% to 15% body weight loss, is a medical emergency. Signs include lethargy, sunken eyes, a very rapid and weak pulse, low blood pressure, and skin that “tents” when pinched and stays raised for several seconds. In infants, the soft spot on the skull may appear noticeably sunken.
What Shifts Your Body’s Water Percentage
Your water percentage isn’t a fixed number. It fluctuates throughout the day and across your lifetime based on several factors:
- Body fat percentage: Fat tissue contains far less water than muscle. Gaining fat lowers your overall water percentage; gaining muscle raises it.
- Age: The gradual loss of lean mass with aging means your body holds proportionally less water decade by decade.
- Hydration status: Even within a single day, your water percentage shifts based on how much you drink, how much you sweat, and how much sodium you consume (since sodium helps regulate how much water your body retains outside cells).
- Fitness level: Athletes and highly active people tend to carry more muscle mass and therefore a higher percentage of body water compared to sedentary individuals of the same weight.
If you’ve ever weighed yourself before and after a long run and noticed a drop of a pound or two, that’s almost entirely water loss, not fat. A liter of sweat weighs about 2.2 pounds, so the scale reflects hydration changes far more than actual tissue changes over short periods.