The human body is roughly 50% to 65% water by weight, though the exact number depends on your age, sex, and body composition. A newborn baby is about 75% water, while an older adult may fall below 50%. For most healthy adults, water accounts for more than half of total body weight.
What Determines Your Body’s Water Percentage
Several factors push your personal number higher or lower within that 50% to 65% range. Fat tissue holds much less water than muscle, so people with more muscle mass tend to have a higher overall water percentage. This is one reason adult men generally land at the higher end of the range and women toward the lower end: on average, men carry more lean muscle relative to body fat.
Hydration status matters too. If you’re dehydrated from exercise, illness, or simply not drinking enough, your total body water dips temporarily. But the biggest single factor is age.
How Water Content Changes With Age
Newborns are the most water-dense humans. In the first 28 days of life, water makes up about 75% of a baby’s total weight. By the time an infant reaches 12 months, that drops to around 60%. Throughout childhood, total body water continues to shift gradually, reaching adult values around age 12.
Adults between 13 and 64 average about 55% water by weight. After 65, both total body mass and water content decline further. Older adults lose water partly because they tend to lose muscle mass and partly because the body’s ability to conserve water becomes less efficient over time. This is one reason dehydration poses a greater risk in older populations.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. Some organs are surprisingly water-rich, while others contain relatively little.
- Lungs: about 83% water, the highest of any major organ
- Muscles and kidneys: about 79% water
- Brain and heart: about 73% water
- Bones: roughly 31% water
Even bones, which feel completely solid, contain nearly a third water. Your blood is also heavily water-based, which is why losing fluid quickly through sweating or illness can affect blood pressure and circulation so rapidly.
Where the Water Actually Sits
Your body stores water in two main compartments. About 60% of your total body water sits inside your cells, where it serves as the medium for nearly every chemical reaction that keeps you alive. The remaining 40% is outside your cells: in your blood plasma, in the fluid between tissues, and in specialized spaces like the fluid surrounding your brain and spinal cord.
This balance between water inside and outside cells is tightly regulated. Your body constantly adjusts the concentration of salts and other dissolved particles to move water where it’s needed. When that balance is disrupted, whether by dehydration, overhydration, or certain medical conditions, the effects can range from mild fatigue to serious complications.
What All That Water Does
Water isn’t just filling space. It plays active roles in almost every bodily function. It regulates body temperature through perspiration: when you overheat, sweat carries water to your skin surface, and evaporation cools you down. Water also acts as a solvent, dissolving nutrients and carrying them through your bloodstream to the cells that need them. Waste products travel the reverse route, dissolved in water and filtered out through your kidneys.
In your digestive system, water helps move food through the intestines and prevents constipation. It cushions your joints and protects sensitive tissues like your brain and spinal cord. It’s also essential for the chemical reactions that break down food into usable energy. Without adequate water, none of these processes run efficiently.
How Much Water You Cycle Through Daily
Your body doesn’t just hold water in a static pool. You’re constantly losing it and replacing it. On a typical day, you lose water through urine, sweat, breathing, and digestion. The total varies widely based on activity level, climate, and body size, but most adults need to replace roughly 2 to 3 liters per day to stay in balance.
Not all of that needs to come from drinking water directly. Foods, especially fruits and vegetables, contribute a meaningful amount. Cooked grains, soups, and dairy products also contain significant water. For most people, thirst is a reliable signal that it’s time to drink, though older adults sometimes experience a blunted thirst response and may need to be more intentional about fluid intake.
Your body is, in a very real sense, a water-based system. That 50% to 65% figure isn’t just a trivia answer. It reflects how deeply water is woven into every function your body performs, from temperature regulation to nutrient delivery to the chemical reactions happening inside every one of your cells.