Your body is roughly 50% to 75% water by weight, with the exact number depending on your age, sex, and how much body fat you carry. For most healthy adults, the figure lands around 55% to 62%. That single number hides a lot of variation, though, so it’s worth understanding what shifts it and why water matters so much to how your body functions.
Water Percentage by Age and Sex
Children carry the most water relative to their size. Between ages 3 and 10, both boys and girls are about 62% to 63% water. From puberty onward, the numbers diverge. Males hold steady around 62% to 63% through most of adulthood, then drop to about 57% after age 60. Females see an earlier decline, falling to around 55% to 56% during the teenage years and staying in that range through middle age before dropping to roughly 50% after 60.
The primary reason for the sex difference is body composition. Lean tissue (muscle, organs, bone) is about 70% to 75% water. Fat tissue holds only about 10%. Because women on average carry a higher percentage of body fat than men, their overall water percentage is lower. This also explains why two people of the same sex and age can have meaningfully different body water levels if one is leaner than the other.
Where the Water Actually Sits
Water isn’t evenly distributed. Some organs are far more water-dense than you might expect. Your lungs top the list at about 83% water, which makes sense given that they need thin, moist membranes to exchange oxygen and carbon dioxide. Your kidneys and skeletal muscles come in around 79%, while your brain and heart are each roughly 73%. Even your bones contain some water, though far less than soft tissue.
Because muscle is so water-rich, people with more muscle mass carry a higher total body water percentage. This is the same reason athletes and younger adults tend to test at the higher end of the range, while older adults, who gradually lose muscle over the decades, trend lower.
What All That Water Does
Water plays a role in nearly every process your body runs, but three functions stand out as the most critical.
Temperature Control
When you overheat, whether from exercise, hot weather, or illness, your body cools itself by pushing water to the skin’s surface as sweat. As that sweat evaporates, it pulls heat away from your body. This system depends on having enough fluid available. When you’re dehydrated, your blood volume drops and your blood becomes more concentrated. Your body can no longer produce enough sweat to keep your core temperature in check, which is why dehydration in the heat can escalate quickly from discomfort to a medical problem.
Waste Filtering
Your kidneys filter your entire blood supply many times a day, pulling out waste products and excess substances for disposal through urine. This process requires water. There’s a minimum volume of urine your kidneys must produce to clear the waste your body generates, and the maximum rate they can process is about one liter per hour. When water is scarce, the kidneys compensate by concentrating urine more heavily, but this puts extra strain on kidney tissue. Over time, chronically low water intake, especially combined with a high-salt diet, forces the kidneys to work harder than they need to.
Transporting Nutrients and Cushioning Joints
Blood is mostly water, and it serves as the delivery system for oxygen, glucose, vitamins, and minerals to every cell. Water also forms the basis of the fluid that cushions your joints and surrounds your spinal cord and brain. Without adequate hydration, these systems don’t stop working entirely, but they become less efficient.
How Your Body Maintains Its Water Balance
Your kidneys are the main regulators. They constantly adjust how much water they retain or release based on signals from hormones. When your blood pressure drops or your blood becomes too concentrated, a cascade of hormonal signals tells the kidneys to hold onto more water and sodium. When you’ve taken in excess fluid, the kidneys dial back reabsorption and you produce more dilute urine.
This system is remarkably precise. Different sections of the kidney handle different parts of the job. Some segments pull water back into the bloodstream through specialized channels called aquaporins. Others fine-tune sodium and potassium levels. The result is that a healthy body keeps its water percentage within a narrow range day to day, even as your intake fluctuates.
Thirst is the conscious layer on top of this system, but it’s not always perfectly calibrated. Older adults often experience a blunted thirst response, which is one reason dehydration is more common in people over 60.
How Body Water Is Measured
If you’ve ever stepped on a smart scale at home or at a gym, it likely estimated your body water using bioelectrical impedance analysis. This method sends a tiny electrical current through your body. Because water conducts electricity well and fat does not, the resistance the current encounters gives an estimate of how much of your weight is water versus fat.
The gold standard in research settings is isotope dilution, where you drink water containing a harmless tracer and then provide blood or urine samples so scientists can calculate your total body water with high precision. Studies comparing the two methods in healthy adults have found that bioelectrical impedance provides a reasonably accurate estimate, so the number on your smart scale is in the right ballpark, even if it’s not lab-grade.
What Shifts Your Number
Several everyday factors cause your body water percentage to fluctuate, sometimes by a few percentage points within a single day. Exercise and heat exposure increase water loss through sweat. Drinking a large amount of fluid temporarily raises the percentage until your kidneys catch up. High-sodium meals cause your body to retain water, which is why you might feel puffy or weigh slightly more the morning after salty food.
Longer-term shifts come from changes in body composition. Gaining muscle increases your water percentage; gaining fat decreases it. This is why body water percentage is sometimes used as a rough proxy for fitness level, though it’s not precise enough to replace more direct measures of body composition. Aging also reliably lowers the number, both because of gradual muscle loss and because the body’s water-regulation systems become less efficient over time.