The human body is roughly 50 to 80 percent water, with the exact number depending on your age, sex, and how much muscle versus fat you carry. A typical adult male averages around 60 percent, while an adult female averages closer to 50 to 55 percent. That range surprises many people, but it reflects real biological differences in body composition that shift across a lifetime.
Why the Percentage Varies So Much
The single biggest factor driving your body’s water content is the ratio of lean tissue to fat tissue. Muscle holds roughly six times more water per unit of weight than fat does. Lean tissue is about 70 to 80 percent water, while fat tissue is only around 14 percent water. This is why two people of the same weight can have very different total body water levels: someone with more muscle will carry a higher percentage.
This also explains the gap between men and women. Men tend to carry more muscle mass and less body fat, so their overall water percentage runs higher. Women, who on average carry a higher proportion of body fat, fall on the lower end. Neither number is better or worse; they simply reflect different body compositions.
How Water Content Changes With Age
Newborns are the most water-rich humans, with body water making up roughly 75 percent of their weight. That percentage gradually decreases through childhood and stabilizes in adulthood. Then it drops again in older age, primarily because of the natural loss of muscle mass that comes with aging. Less muscle means less tissue capable of storing water.
This decline matters practically. Older adults have a smaller water reserve to draw on, which means they become dehydrated faster and recover from it more slowly. Combined with a weaker thirst signal (the brain’s thirst cue diminishes with age), this makes dehydration one of the more common and underappreciated health risks in people over 65.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. Some organs are surprisingly water-dense, while others are relatively dry. Data from the U.S. Geological Survey breaks it down:
- Lungs: about 83 percent water
- Kidneys: 79 percent
- Brain: 73 percent
- Heart: 73 percent
- Bones: 31 percent
Blood is in a category of its own. Plasma, the liquid portion that makes up a little over half of your blood volume, is about 92 percent water. That high water content is what allows plasma to transport nutrients, hormones, and waste products efficiently throughout the body.
Where the Water Sits Inside Your Body
Your body’s water isn’t sloshing around freely. About 60 percent of it sits inside your cells, forming what physiologists call the intracellular fluid. This is the water that keeps cells plump, supports chemical reactions, and provides the medium for everything from energy production to DNA replication. The remaining one-third exists outside of cells as extracellular fluid, which includes the liquid portion of blood, the fluid between cells, and specialized fluids like cerebrospinal fluid and the moisture lining your joints.
Your body tightly regulates the balance between these two compartments. When you become dehydrated, water shifts between compartments to maintain blood pressure and keep critical organs functioning. This is why mild dehydration can cause symptoms like fatigue and poor concentration before any obvious signs of thirst kick in: the redistribution of water prioritizes survival over comfort.
What All That Water Actually Does
Water serves as the body’s primary solvent, meaning it dissolves and transports the molecules your cells need to function. Nutrients from food get carried to tissues through water-based blood plasma. Waste products travel through the bloodstream to the kidneys, which filter them out in a process that itself depends on adequate water. Your kidneys, at 79 percent water, process roughly 120 to 150 liters of fluid per day to produce about one to two liters of urine.
Temperature regulation is another critical role. When your body overheats, you sweat, and the evaporation of that water from your skin pulls heat away. Water also absorbs and distributes heat internally, preventing dangerous temperature spikes in active organs like the heart and brain. It cushions the brain inside the skull, lubricates joints, and keeps the mucosal linings of your lungs and digestive tract moist enough to function.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that estimates body water, it was using a technique called bioelectrical impedance. The scale sends a tiny electrical current through your body and measures how easily it passes. Water conducts electricity well, so more resistance suggests more fat (which holds less water) and less lean tissue.
These consumer devices give rough estimates. The gold standard in clinical and research settings is isotope dilution. A person drinks a small dose of water labeled with a harmless tracer, most commonly deuterium (a stable, non-radioactive form of hydrogen). After a few hours, researchers measure how diluted the tracer has become in a sample of blood, urine, or saliva. The degree of dilution reveals the total volume of water in the body with high precision.
Practical Implications of Your Water Percentage
Knowing that your body is more than half water puts daily hydration into perspective. Losing just 1 to 2 percent of your body water can impair cognitive performance, reduce physical endurance, and trigger headaches. A 150-pound person who is 60 percent water carries about 90 pounds (roughly 40 liters) of water, so a 2 percent loss is less than a liter, easily lost through a hard workout or a few hours in the heat without drinking.
Your body composition also affects how alcohol, medications, and anesthesia distribute through your system. Someone with a higher water percentage dilutes water-soluble substances more effectively than someone with more body fat. This is one reason the same dose of alcohol produces a higher blood alcohol concentration in someone with less lean mass, even at the same body weight. It is also why dosing guidelines sometimes account for sex and age: both are proxies for estimated body water.