Water makes up roughly 60% of an adult human body by weight. For a 70-kilogram (154-pound) person, that translates to about 42 liters, or roughly 11 gallons. But that number shifts significantly depending on your age, sex, and body composition.
Water Percentage by Age and Sex
Newborns carry the highest proportion of water, around 75% of their body weight. That percentage gradually declines through childhood and into adulthood. Adult men average about 60% water, while adult women typically fall closer to 50-55%. The difference comes down to body composition: women generally carry more body fat, which holds far less water than lean tissue. In older adults, total body water drops further as muscle mass decreases with age, often settling around 45-50%.
Why Body Fat Changes the Equation
Lean tissue and fat tissue hold dramatically different amounts of water. Adipose (fat) tissue contains only about 14% water, while fat-free mass is roughly 80% water. That means lean tissue holds approximately six times more water per unit of weight than fat does. This single fact explains most of the variation you see between individuals. Two people who weigh the same can differ by several liters of total body water simply because one carries more muscle and the other carries more fat.
Where Water Lives Inside Your Body
Not all of your body’s water sits in one place. About two-thirds of it is inside your cells, forming what physiologists call intracellular fluid. The remaining third is outside cells, in your blood plasma, the fluid between tissues, and specialized compartments like the fluid around your brain and spinal cord. This is sometimes called the 60-40-20 rule: 60% of body weight is water, 40% is intracellular, and 20% is extracellular.
Different organs hold strikingly different concentrations. The lungs are the most water-rich organ at about 83%. The brain and heart both sit around 73%. Muscles and kidneys come in at roughly 79%, skin at 64%, and even bones, which seem completely solid, are 31% water.
What All That Water Actually Does
Water isn’t just filling space. It performs several functions your body depends on constantly.
Your body uses water to regulate temperature. When you overheat, blood carries warmth from deep tissues to the skin surface, and sweat evaporates to pull that heat away. This is the most effective cooling mechanism you have, and it relies entirely on having enough water available to produce sweat.
Water also acts as a solvent for nutrients. Carbohydrates, vitamins, minerals, and oxygen all dissolve in water so they can travel through your bloodstream and enter cells. The same system works in reverse for waste: water carries metabolic byproducts to your kidneys and out through urine.
Inside joints, water is a key component of the fluid that cushions and lubricates movement. It also surrounds the brain, acting as a shock absorber that protects it from impact against the skull. Every chemical reaction happening in your cells takes place in a water-based environment.
How Much Water You Lose Each Day
Your body constantly cycles through its water supply. In a temperate climate, a typical adult loses water through four main routes every day:
- Urine: 1 to 2 liters, the largest and most variable source of loss
- Skin evaporation: about 450 milliliters, even without visible sweating
- Breathing: 250 to 350 milliliters lost as moisture in exhaled air
- Stool: roughly 200 milliliters
That adds up to somewhere between 2 and 3 liters on a quiet day. Physical activity, heat, altitude, and illness can push losses significantly higher. Heavy exercise in hot weather can increase sweat losses alone to over a liter per hour.
How Much You Need to Replace
General guidelines suggest that healthy adults need about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, with the higher end typical for men. That number includes all fluid sources: drinking water, other beverages, and the water contained in food. Most fruits and vegetables are 80-95% water by weight, and food typically contributes about 20% of your daily intake without you thinking about it.
Your actual needs shift with exercise, climate, body size, and health status. Thirst is a reasonable guide for most healthy people, though it tends to lag slightly behind actual fluid needs during intense activity or in very hot conditions.
How Body Water Is Measured
If you’ve ever stepped on a “smart” scale that estimates body water, it likely used bioelectrical impedance analysis. This method sends a small electrical current through your body and measures resistance. Because water conducts electricity well, the signal travels faster through hydrated lean tissue than through fat. Combined with your height and weight, the measurement can estimate total body water with a margin of error around 2 to 3 liters.
The gold standard method uses isotope dilution, where you drink a small amount of water tagged with a traceable hydrogen isotope (deuterium). After it distributes throughout your body, a blood or saliva sample reveals exactly how much water is present. This approach is highly accurate but mostly reserved for research settings. Bioelectrical impedance works well enough for clinical and personal use, provided the equations are calibrated for your age and population group.