About 60% of the average adult human body is water. That number shifts depending on your age, sex, and body composition, but for most people it falls somewhere between 45% and 75%. Water is the single largest component of your body, outweighing bone, muscle, and fat combined.
How Age and Sex Change the Number
Newborns are the most water-dense humans, with roughly 75% of their body weight coming from water. That percentage drops steadily through childhood and into adulthood. By the time you’re a young adult, it settles around 60% for men and closer to 50–55% for women. The difference comes down to body composition: women typically carry a higher percentage of body fat, and fat tissue holds far less water than lean tissue.
As you age past 60, total body water continues to decline. Older adults often fall below 50%, partly because muscle mass decreases and body fat increases. This is one reason older adults are more vulnerable to dehydration: there’s simply less of a water reserve to draw from.
Why Body Fat Matters So Much
The gap between lean tissue and fat tissue is dramatic. Adipose (fat) tissue is only about 14% water, while the rest of your body, your adipose-free mass, is roughly 80% water. That means lean tissue holds about six times more water per unit of weight than fat does. Two people who weigh the same can have very different total body water percentages if one carries significantly more muscle and the other carries more fat.
This is also why athletes and people with more muscle mass tend to land on the higher end of the body water range, sometimes approaching 65%, while someone with a higher body fat percentage might be closer to 45%.
Where the Water Actually Sits
Not all of your body water sloshes around freely. About two-thirds of it, roughly 40% of your total body weight, is locked inside your cells. This is the water that keeps cellular chemistry running. The remaining one-third, about 20% of body weight, is outside your cells: in your blood, in the fluid between tissues, and in places like your spinal canal and the chambers of your eyes. Physiologists call this the 60-40-20 rule.
Your blood plasma, the liquid portion that carries red and white blood cells, is about 90% water. Plasma makes up roughly 55% of your blood’s total volume, which is why losing even a modest amount of water through sweating or illness can noticeably affect blood pressure and circulation.
Water Content by Organ
Some organs are far more water-rich than others. According to data compiled by the U.S. Geological Survey, here’s how different parts of the body compare:
- Lungs: 83% water
- Muscles and kidneys: 79% water
- Brain and heart: 73% water
- Skin: 64% water
- Bones: 31% water
Even bones, the densest and hardest structures in your body, are nearly a third water. The deeper layer of your skin (the dermis) is actually more water-rich than the 64% whole-skin average suggests, measuring around 72–74% water in studies using specialized imaging. The drier outer layer pulls the average down.
What All That Water Does
Water isn’t just filling space. It carries nutrients and oxygen to your cells, dissolves minerals so your body can absorb them, and flushes waste products out through your kidneys. It lubricates your joints, cushions your brain and spinal cord, and plays a central role in temperature regulation. When you overheat, your body pushes water to the skin’s surface as sweat, and evaporation cools you down. Without enough water, every one of these processes becomes less efficient.
Even mild dehydration, losing just 1–2% of your body water, can affect concentration, energy levels, and physical performance. Your body is constantly losing water through breathing, sweating, and digestion, so maintaining that 60% balance requires consistent intake throughout the day.
How Much Water You Need Daily
The National Academies of Sciences sets the adequate intake for total water (from all beverages and food combined) at 3.7 liters per day for men and 2.7 liters per day for women. Those recommendations hold steady from age 19 through 70 and beyond. “Total water” includes water from food, which typically accounts for about 20% of daily intake. So you don’t need to drink 3.7 liters of plain water; a significant portion comes from fruits, vegetables, soups, and other foods.
Your actual needs shift with activity level, climate, and health status. Heavy exercise, hot weather, fever, and illness all increase water loss and push requirements higher. Thirst is a reasonable guide for most healthy adults, though it becomes less reliable with age, which is another reason older adults face higher dehydration risk.