The average adult human body is roughly 55 to 60% water by weight. For a 70-kilogram (154-pound) man, that translates to about 40 liters (10.5 gallons) of total body water. Women carry slightly less, typically 50 to 55% of their body weight, largely because female bodies tend to have a higher proportion of fat tissue, which holds less water than muscle.
How Body Water Changes With Age and Sex
Babies are the most water-rich humans. During early childhood, total body water sits around 62% of body weight for both boys and girls. That number stays relatively equal between sexes until puberty, when girls’ body water percentage drops to about 55% while boys’ remains closer to 60%.
For most of adulthood, those percentages hold steady. After age 60, both sexes see a gradual decline. Men drop to roughly 57%, and women to about 50%. This happens because older adults tend to lose muscle mass and gain fat tissue, and fat cells contain far less water than muscle cells. It’s one reason older adults are more vulnerable to dehydration.
Where All That Fluid Actually Sits
Your body doesn’t store water in one big reservoir. It’s divided into two main compartments, and knowing the split helps explain why dehydration affects your whole body so quickly.
About two-thirds of your total body water, roughly 25 liters in an average adult male, is inside your cells. This fluid powers the chemical reactions that keep cells alive, from energy production to protein building. The remaining third, about 15 liters, sits outside your cells. That extracellular fluid breaks down further into two categories: the liquid portion of your blood (plasma) and interstitial fluid, which fills the tiny spaces between cells throughout your tissues.
Your body constantly shuttles water between these compartments using electrically charged minerals called electrolytes. Sodium is the dominant one outside your cells, while potassium dominates inside. When a sodium ion enters a cell, a potassium ion leaves, and this back-and-forth exchange controls how much water each compartment holds at any given moment. Chloride, the second most abundant ion in the body, also plays a key role in maintaining this balance.
Water Content Varies Dramatically by Organ
Not every part of your body holds the same amount of water. Your lungs are the most water-rich major organ at about 83%. The brain and heart are both around 73%. Muscles and kidneys come in at 79%. Even your bones, which feel completely solid, contain about 31% water. Skin sits around 64%.
These differences explain why dehydration produces such varied symptoms. Your brain’s high water content is one reason even mild fluid loss can cause headaches, difficulty concentrating, and fatigue. Your kidneys’ dependence on water is why urine color is such a reliable hydration indicator.
Blood Volume and Plasma
An average adult carries about 5.5 liters (roughly 1.5 gallons) of blood. Plasma, the liquid portion, makes up about 55% of that volume and is itself more than 90% water. Plasma serves as the transport highway for nutrients, hormones, and waste products moving between organs. When you’re dehydrated, plasma volume drops, your blood becomes thicker, and your heart has to work harder to circulate it. That’s why dehydration often shows up as a faster heart rate before you even feel thirsty.
Specialized Fluids You Rarely Think About
Beyond the major compartments, your body maintains small but critical pockets of specialized fluid. Adults carry about 150 milliliters (5 ounces) of cerebrospinal fluid, with most of it (125 milliliters) cushioning the brain and spinal cord and the rest filling the brain’s internal chambers. Your body produces 400 to 600 milliliters of this fluid every day, constantly recycling and replacing it.
Synovial fluid lubricates your joints, though each joint holds only a tiny amount. Your eyes contain aqueous humor that maintains their shape and nourishes internal structures. Pleural fluid surrounds your lungs, reducing friction as they expand and contract with each breath. Individually, these volumes are small. Collectively, they’re essential to keeping your body’s mechanical and protective systems running.
How Much Fluid You Lose and Replace Daily
Your body cycles through a surprising amount of water every 24 hours. Urine accounts for the largest loss at roughly 1,500 milliliters per day. You also lose water through breathing, sweating, and bowel movements. In total, a sedentary adult in a temperate climate loses about 2 to 3 liters daily, and that number climbs significantly with exercise, heat, or illness.
To keep up, most healthy adults need about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day. That includes water from all sources: drinking water, other beverages, and the water naturally present in food. Fruits, vegetables, soups, and even cooked grains contribute meaningfully to your daily intake. For most people, drinking when thirsty and paying attention to urine color (pale yellow signals good hydration) is a more practical guide than hitting a specific number.
How Total Body Water Is Measured
If you’ve ever wondered how scientists arrived at these numbers, the gold standard is a technique called isotope dilution. You drink a small, precisely weighed dose of water labeled with a traceable form of hydrogen called deuterium. After a few hours, the isotope spreads evenly through your body water, and a saliva sample reveals how diluted it became. The degree of dilution tells researchers your exact total body water volume. The whole process takes about five hours.
A quicker, more common method is bioelectrical impedance analysis, which many body composition scales use. It sends a tiny electrical current through your body. Since water conducts electricity well and fat does not, the resistance your body offers gives an estimate of how much water you’re carrying. It’s less precise than isotope dilution but widely available and completely painless.
Once total body water is known, clinicians can estimate lean body mass by dividing total body water by 0.732, since lean tissue is about 73.2% water. The remainder is fat mass. This is one reason hydration status matters for accurate body composition testing: if you’re dehydrated, these calculations will overestimate your body fat percentage.