Water makes up about 60% of an adult’s total body weight. For a 150-pound person, that’s roughly 90 pounds of water, or about 11 gallons. But that number isn’t fixed. It shifts based on your age, sex, and how much body fat you carry.
How Body Water Varies by Age and Sex
Babies are the most water-dense humans. Water accounts for roughly 60% to 70% of an infant’s total body weight, largely because they carry proportionally more lean tissue and less fat. As children grow and body composition changes, that percentage gradually settles into adult ranges.
For adult men, total body water typically falls around 60% of body weight. For adult women, it’s closer to 50% to 55%. The gap comes down to body composition: women on average carry a higher percentage of body fat, and fat tissue holds considerably less water than muscle. If two people weigh the same but one has more muscle, that person will have a higher water percentage.
As you age past 60 or so, total body water declines further. Older adults tend to lose muscle mass and gain fat tissue, both of which reduce the body’s water proportion. Research in the American Journal of Clinical Nutrition confirms that this decline is well documented in large population studies, and it appears to be driven mainly by a drop in water inside cells rather than in the fluid surrounding them. This shift is one reason older adults are more vulnerable to dehydration, even mild cases.
Where Water Lives in Your Body
Water isn’t distributed evenly. Some organs are surprisingly water-rich, while others you might assume are “dry” still contain a significant amount. According to the U.S. Geological Survey, here’s how it breaks down:
- Lungs: 83% water
- Muscles and kidneys: 79%
- Brain and heart: 73%
- Skin: 64%
- Bones: 31%
Your lungs top the list because they need a thin layer of moisture to facilitate gas exchange with every breath. Muscles rank high because they’re metabolically active tissue that depends on water for energy production and waste removal. Even bones, which feel solid and rigid, are nearly one-third water by weight. Blood, though not listed above, is about 90% water and serves as the body’s primary transport system for nutrients, oxygen, and waste products.
Why Body Fat Changes the Equation
The single biggest factor that shifts your body water percentage (beyond age) is how much fat tissue you carry. Fat cells store energy as lipids, which are hydrophobic, meaning they repel water. Muscle cells, by contrast, are packed with water-dependent structures that power movement and metabolism. A lean, muscular person can have a body water percentage well above 60%, while someone with a higher body fat percentage may fall below 50%.
This is why two people of the same weight can have very different total body water. It also explains why athletes and highly active individuals tend to have higher water percentages, and why body water calculations based on weight alone are only rough estimates.
How Much Water You Lose Each Day
Your body is constantly cycling water out. The biggest exit route is your kidneys: depending on your fluid intake and what your body needs, you can excrete anywhere from about half a liter to over 10 liters of urine per day, though most people fall somewhere around 1 to 2 liters. Beyond urine, you lose roughly a liter of water daily just by breathing and through evaporation from your skin, even without visible sweating. Exercise, hot weather, and fever all push that number higher. A small amount also leaves through your digestive tract, though under normal circumstances this contribution is minimal.
All told, a typical adult loses somewhere around 2 to 3 liters of water per day under normal conditions. That’s a meaningful chunk of your total supply, which is why consistent fluid intake matters so much.
How Much You Need to Replace
General recommendations suggest that healthy adults need roughly 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day. That range, cited by the Mayo Clinic, includes all sources: drinking water, other beverages, and the water content of food. Most people get about 20% of their daily water from food alone, particularly from fruits, vegetables, soups, and other high-moisture items.
The lower end of that range applies more to women and smaller-bodied individuals, while the higher end corresponds to men and people who are more physically active. Climate plays a role too. If you live in a hot or dry environment, or if you exercise regularly, your losses are higher and your intake needs to match. Pregnancy and breastfeeding also increase fluid requirements significantly.
Signs Your Water Balance Is Off
Your body is remarkably good at regulating its own water balance through thirst signals, hormone adjustments, and kidney function. But that system isn’t perfect, and it becomes less reliable with age. Early signs of mild dehydration include dark yellow urine, dry mouth, fatigue, and headaches. You might also notice reduced concentration or dizziness when standing up quickly.
On the other end, drinking far more water than your body can process in a short time can dilute the sodium in your blood, a condition called overhydration. This is rare in everyday life but can occur during endurance exercise when people drink aggressively without replacing electrolytes. For most people, paying attention to thirst and checking urine color (pale yellow is the target) provides a reliable, practical gauge of hydration status without needing to measure anything precisely.