The average adult human body is about 55 to 60% water by weight. That number shifts depending on your sex, age, and how much muscle versus fat you carry. For most adult men, water accounts for roughly 60% of body weight, while for most adult women it’s closer to 52 to 55%.
Why the Number Varies Between People
The single biggest factor influencing your body’s water percentage is body composition. Lean tissue (muscle, organs, skin) holds dramatically more water than fat. Muscle and other lean tissue is about 70 to 80% water by weight, while fat tissue is only about 14 to 20% water. That means a lean, muscular person carries a higher proportion of water than someone with more body fat, even if they weigh exactly the same.
This difference in tissue composition is the main reason men tend to have a higher water percentage than women. Men on average carry more muscle mass and less body fat, so their bodies hold proportionally more water. Two people of the same sex and age can still differ by 10 percentage points or more depending on fitness level and body fat.
How Body Water Changes With Age
Newborns are remarkably watery. From birth to six months, a baby’s body is about 74% water on average, with a range as wide as 64 to 84%. By six months to a year, that average drops to around 60%, and it stays near 60% through childhood.
The decline continues into older adulthood as muscle mass gradually decreases and body fat tends to increase. Men over 51 average about 56% water (range: 47 to 67%), while women over 51 average about 47% (range: 39 to 57%). This lower water reserve in older adults is one reason dehydration can become a more serious concern with age.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. According to data from the U.S. Geological Survey, your lungs are the most water-rich major organ at about 83%. Your kidneys come in at 79%, and both your brain and heart sit around 73%. Even your bones, which feel solid and dry, are about 31% water.
These numbers help explain why dehydration affects thinking and heart function so quickly. When the organs that depend most on water lose even a small percentage, performance drops.
Where All That Water Lives
About two-thirds of your total body water sits inside your cells. This fluid is the medium where most of your body’s chemical reactions take place. The remaining one-third is outside your cells: in your blood plasma, in the fluid between cells, and in specialized compartments like cerebrospinal fluid and the fluid inside your eyes and joints.
Your body constantly moves water between these compartments to maintain blood pressure, deliver nutrients, and regulate temperature. Even small shifts in this balance trigger thirst, changes in urine concentration, and hormonal signals that tell your kidneys to retain or release water.
What Dehydration Actually Looks Like
Because so much of your body weight is water, losing even a small percentage has noticeable effects. In children, losing up to 3% of body weight as water is considered mild dehydration, 6% is moderate, and 9% is severe. Infants can tolerate slightly higher percentages before reaching the same severity levels, but they also lose water faster relative to their size.
For adults, a loss of just 1 to 2% of body weight from sweating or inadequate fluid intake is enough to impair concentration, increase fatigue, and trigger headaches. By the time you feel distinctly thirsty, you’re typically already mildly dehydrated.
How Much Water You Need Daily
The National Academy of Medicine recommends a total daily fluid intake of about 13 cups (104 ounces) for men and 9 cups (72 ounces) for women. That includes all beverages, not just plain water. Roughly 20% of your daily water intake typically comes from food rather than drinks, especially from fruits and vegetables like cucumbers, lettuce, bell peppers, celery, berries, and melons.
Your actual needs shift with activity level, climate, and body size. If you’re sweating heavily from exercise or heat, you’ll need considerably more than the baseline recommendation. Pregnancy and breastfeeding also increase fluid requirements.