The human body is about 60% water by weight in a typical adult. That number isn’t fixed, though. It shifts depending on your age, sex, body composition, and even the time of day. A newborn can be as much as 75% water, while an older adult might drop closer to 50%.
How the Percentage Changes With Age and Sex
Infants and young children carry the most water relative to their size. Children’s bodies are roughly 60% to 70% water, largely because they have proportionally more lean tissue and less fat. As people grow into adulthood, that range typically settles around 60% for men and closer to 50% to 55% for women. The difference comes down to body fat: fat tissue holds less water than muscle, and women on average carry a higher percentage of body fat than men.
As you age past middle adulthood, body water percentage gradually declines. You lose muscle mass and gain fat tissue over the decades, both of which reduce total body water. This is one reason older adults are more vulnerable to dehydration. Their margin is thinner to begin with, and the sensation of thirst often dulls with age.
Where Water Lives Inside Your Body
Not all of that water sloshes around freely. About two-thirds of your body’s water sits inside your cells, in what’s called intracellular fluid. This is the water that fills every cell in your muscles, organs, and brain, keeping the chemical reactions of life running. The remaining one-third is extracellular fluid: the water in your blood plasma, the fluid between your cells, and smaller reservoirs like cerebrospinal fluid and the moisture in your eyes.
For a 70-kilogram (154-pound) man carrying about 42 liters of total body water, roughly 28 liters would be inside cells and 14 liters outside them. Your body works constantly to maintain this balance. When the ratio shifts too far in either direction, you feel the effects quickly: swelling, confusion, muscle cramps, or drops in blood pressure.
Water Content Varies Widely by Organ
Some parts of your body are far more water-rich than others. The brain and heart are both about 73% water. Your lungs are even higher, at roughly 83%. Skin sits around 64%, muscles and kidneys near 79%. Even your bones, which feel solid and dry, are about 31% water. That skeletal water plays a role in maintaining bone flexibility and supporting the marrow inside, where blood cells are produced.
The organs that do the most metabolic work tend to hold the most water, because water is the medium in which nearly every biochemical reaction takes place. Your liver, kidneys, and brain all depend on a steady supply.
What All That Water Actually Does
Water isn’t just filling space. It serves as the solvent for virtually every process in your body. Nutrients from the food you eat dissolve in water so they can be absorbed through intestinal walls and delivered to cells via your bloodstream. Waste products travel the same way, dissolved in blood plasma until your kidneys filter them out.
Temperature regulation depends heavily on water. When you overheat, your body pushes water to the skin’s surface as sweat. As that sweat evaporates, it pulls heat away from your body. This cooling system is remarkably effective, but it only works if you have enough water to spare. Dehydration compromises it quickly.
Water also cushions your brain and spinal cord, lubricates your joints, and helps maintain blood volume and pressure. It participates directly in chemical reactions like the breakdown of food molecules during digestion. In short, there is no bodily function that works without it.
How Body Composition Shifts the Number
Two people of the same age and sex can have noticeably different body water percentages based on how much muscle versus fat they carry. Muscle tissue is about 79% water, while fat tissue holds only about 10% to 20%. A lean, muscular person will have a higher total body water percentage than someone of the same weight with more body fat. This is why athletes sometimes measure above 60%, and why people with obesity often measure below the typical range for their age group.
Research confirms significant differences in total body water across race and sex groups as well, meaning a single “60%” figure is a useful average but not a universal standard. Your individual number depends on the specific composition of your body.
How Body Water Is Measured
If you’ve ever stepped on a smart scale that claims to measure your body water percentage, it was using a technique called bioelectrical impedance analysis (BIA). The scale sends a small, painless electrical current through your body. Water conducts electricity well, so the device estimates total body water based on how easily the current passes through. These consumer devices are portable, safe, and fast, but they come with a margin of error of roughly 3% to 8% for total body water.
More advanced versions use multiple electrical frequencies to distinguish between the water inside your cells and the water outside them, which gives a more detailed picture. These are common in clinical settings and tend to be somewhat more accurate for specific measurements.
The gold standard in research is isotope dilution, where you drink water containing a harmless tracer molecule and provide blood or urine samples afterward. By measuring how the tracer gets diluted, researchers can calculate your total body water with high precision. This method is too time-consuming for routine use, but it’s the benchmark against which other tools are validated.
How Much Water You Lose Daily
Your body loses water continuously. On an average day without heavy exercise, a typical adult loses about 2.5 liters through urine, sweat, breathing, and bowel movements. In hot weather or during intense physical activity, losses can double or more. You replace that water through drinking, eating (most foods contain significant water, especially fruits and vegetables), and a small amount generated internally when your body metabolizes food for energy.
Losing just 1% to 2% of your body water can cause noticeable thirst, fatigue, and reduced concentration. By the time you’ve lost 5% or more, you’re in serious dehydration territory, with symptoms like rapid heartbeat, dizziness, and confusion. Because older adults start with less total body water and may not feel thirst as reliably, they reach dangerous levels faster.