The average adult human body is about 50 to 60 percent water by weight. For a 150-pound person, that translates to roughly 10 to 11 gallons of water spread across every organ, tissue, and cell. The exact percentage varies depending on your age, sex, and body composition, but water is by far the largest single component of the human body.
How the Percentage Changes With Age and Sex
Newborns are the most water-rich humans. Infants up to six months old average about 74% water, with a range of 64 to 84%. That high percentage drops steadily through childhood and into adulthood as the body accumulates more fat and bone mass relative to water.
Adult men between 19 and 50 average about 59% water, while adult women in the same age range average about 50%. The gap comes down largely to body fat: fat tissue holds less water than muscle, and women on average carry a higher percentage of body fat. A very lean woman may have a water percentage in the upper 50s, while a man with more body fat might fall into the low 40s. The ranges overlap quite a bit.
After age 50, total body water declines further. Men over 51 average around 56%, and women over 51 average about 47%. This gradual decline happens because older adults tend to lose muscle mass and gain fat tissue, and the kidneys become somewhat less efficient at conserving water.
Where the Water Actually Sits
Not all of your body’s water sloshes around freely. About two-thirds of it is locked inside your cells, forming the medium where most of your body’s chemical reactions take place. The remaining third sits outside your cells: in your blood, in the fluid between tissues, and in smaller compartments like the fluid around your brain and spinal cord. This is sometimes called the 60-40-20 rule. Roughly 60% of body weight is water, 40% is water inside cells, and 20% is water outside cells.
That distribution matters because your body works hard to keep the balance between these compartments stable. When you’re dehydrated, water shifts between compartments in a tightly regulated process that affects everything from blood pressure to how well your cells function.
Water Content of Individual Organs
Some organs hold far more water than you might expect. According to the U.S. Geological Survey, the lungs are the most water-rich major organ at about 83%. The kidneys come next at 79%. The brain and heart are both about 73% water. Even bones, which feel completely solid, contain a meaningful amount of water.
Blood is a useful illustration. The liquid portion of blood, called plasma, is about 92% water. Plasma makes up roughly 55% of your total blood volume, with the rest being red blood cells, white blood cells, and platelets. So blood as a whole is mostly water by a wide margin, which is why dehydration thickens blood and makes the heart work harder to pump it.
What All That Water Does
Water isn’t just filler. It plays active roles in nearly every process that keeps you alive. It carries nutrients and oxygen to your cells through the bloodstream. It regulates your body temperature through sweating and the expansion of blood vessels near the skin. It lubricates your joints, cushions your brain and spinal cord, and keeps the tissues in your eyes, nose, and mouth moist.
Your kidneys and liver depend on water to flush waste products out of the body. Water also acts as a solvent, dissolving minerals and other nutrients so your cells can actually absorb and use them. Without adequate water, these processes slow down or become less efficient well before you feel seriously ill.
How Much Water You Lose Every Day
Your body constantly loses water through several routes, even when you’re not exercising or sweating noticeably. The kidneys produce anywhere from about half a liter to over 10 liters of urine per day, depending on how much you drink and what your body needs to excrete. For most people on a normal diet, urine output falls somewhere around 1 to 2 liters daily.
On top of that, you lose close to a liter each day just through evaporation from your skin and moisture in the air you exhale. This “invisible” water loss happens around the clock, even in cool weather. A small additional amount leaves through bowel movements, though that’s typically minimal unless you’re dealing with diarrhea or vomiting.
All told, a sedentary adult in a temperate climate loses roughly 2 to 3 liters of water per day. Physical activity, heat, illness, and altitude can push that number significantly higher.
What Happens When Water Levels Drop
Dehydration is classified by how much body weight you’ve lost in fluid. In children, losing up to 3% of body weight is considered mild dehydration. At that level, you’re still alert and active, and your body is compensating well. You might feel thirsty and notice your mouth is slightly dry, but vital signs stay normal.
At moderate dehydration (around 6% of body weight in children, 6 to 10% in infants), the body starts showing clearer stress signals: a faster pulse, faster breathing, dry mouth, reduced tears, and noticeably less urine output. You feel irritable and intensely thirsty. The body is pulling water from less critical areas to protect blood volume and organ function.
Severe dehydration, at 9% or more of body weight in children, is a medical emergency. Pulse becomes very fast and weak, blood pressure drops, eyes appear sunken, and the person may become lethargic or unresponsive. Urine output drops to almost nothing. At this stage the body can no longer compensate, and organ damage becomes a real risk.
How Body Water Is Measured
If you’ve ever stepped on a “smart” bathroom scale that claims to measure your body water percentage, it was using a technology called bioelectrical impedance analysis, or BIA. These devices send a tiny electrical current through your body and estimate water content based on how easily the current travels (water conducts electricity well, fat does not).
BIA is convenient but only moderately accurate. In research comparisons, single-frequency BIA devices estimated total body water within about 1 liter of the gold-standard method, which involves drinking a small amount of labeled water and measuring how it dilutes in the blood over several hours. That’s close enough to be useful for tracking trends over time, but not precise enough for clinical decisions. Dual-frequency BIA devices, which some higher-end scales use, actually performed worse for total body water in one study, underestimating it by about 5 liters compared to the dilution method.
For most people, the exact number matters less than the trend. If your body water percentage is dropping over weeks or months, it may reflect changes in muscle mass, hydration habits, or overall health worth paying attention to.