Water makes up roughly 60% of the adult human body, though the exact number depends on your age, sex, and body composition. A newborn is about 74% water, while an older adult may be closer to 47-56%. That single “60%” figure you’ll see quoted everywhere is really just a midpoint in a wide range.
Average Body Water by Age and Sex
The percentage of water in your body shifts substantially over a lifetime. Babies are born with the highest proportion: an average of 74%, with a range of 64 to 84%. All that water supports the rapid cell growth happening in the first months of life.
By adulthood, the numbers settle and split along sex lines. Adult men ages 19 to 50 average about 59% water (range: 43 to 73%), while adult women in the same age group average about 50% (range: 41 to 60%). The gap exists largely because women tend to carry a higher percentage of body fat, and fat tissue holds less water than muscle. Someone with more muscle mass will have a higher total body water percentage regardless of sex.
After age 50, the numbers dip again. Men over 51 average 56%, and women over 51 average 47%. This decline reflects the gradual loss of lean muscle tissue that comes with aging, along with changes in how the body regulates fluid balance.
Where the Water Actually Sits
Not all of your body’s water is in one place. About 60% of it is stored inside your cells, where it serves as the medium for nearly every chemical reaction keeping you alive. The remaining roughly 40% lives outside cells: in your blood plasma, in the fluid between cells, and in smaller reservoirs like cerebrospinal fluid and the fluid inside your eyes.
Your body constantly moves water between these compartments. Through a process called osmosis, water flows passively from areas with lower concentrations of dissolved particles to areas with higher concentrations. This lets the larger fluid reservoirs inside and around your cells act as a buffer to protect the smaller, more critical volume of water in your bloodstream from running low.
Water Content of Individual Organs
Some organs are far more water-dense than you’d expect. The lungs top the list at about 83% water, which makes sense given that they need to stay moist to exchange oxygen and carbon dioxide efficiently. The brain and heart are each about 73% water. The kidneys, which filter your entire blood supply dozens of times a day, are roughly 79% water.
Even your skeleton contains water. Bones are about 31% water, a number that surprises most people who think of bone as dry, solid material. In reality, bones are living tissue with blood vessels, marrow, and a mineral matrix that all require water to function.
What Water Does in the Body
Water isn’t just filling space. It’s the solvent that allows nutrients, hormones, and waste products to travel through your bloodstream. It cushions your brain and spinal cord, lubricates your joints, and helps regulate your core temperature. When you overheat, your body pushes water to the skin’s surface as sweat; as it evaporates, it carries heat away.
Water also plays a structural role. Cells maintain their shape partly through internal water pressure. Dehydrate a cell and it shrinks, disrupting the delicate machinery inside. This is one reason even mild dehydration can cause fatigue, poor concentration, and headaches well before you feel dangerously thirsty.
How Your Body Balances Water
Your body runs a tightly controlled system to keep water levels stable. When water drops too low, nerve centers deep in the brain trigger the sensation of thirst. At the same time, the pituitary gland releases a hormone called vasopressin (also known as antidiuretic hormone), which signals the kidneys to hold onto more water and produce less urine. When you have excess water, vasopressin secretion drops, and the kidneys let more water pass through as urine.
Sodium plays a key role in this balancing act. When sodium levels rise (from eating salty food, for example), thirst kicks in and drives you to drink. When sodium levels fall too low, the kidneys excrete more water to bring the ratio back into line. This constant back-and-forth keeps the concentration of dissolved particles in your blood remarkably stable.
How Much Water You Lose Each Day
A healthy adult loses roughly 2,550 milliliters of water per day through four main routes. Urine accounts for the largest share at about 1,500 ml. Insensible losses, the water that evaporates from your skin and exits your lungs with every breath, add up to around 900 ml. Feces account for about 100 ml, and normal light sweating (not exercise-related) adds another 50 ml or so. Heavy exercise, hot weather, illness with fever or diarrhea, and high altitude all push these numbers significantly higher.
To replace those losses, current guidelines suggest that healthy adults take in about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, according to the Mayo Clinic. That total includes water from food, which typically supplies about 20% of your daily intake. Fruits, vegetables, soups, and even coffee all count toward the total.
Why Your Number May Differ
The wide ranges in total body water percentages reflect real biological variation. Body fat is the single biggest factor: fat tissue is only about 10-20% water, while muscle tissue is closer to 75%. Two people of the same age and sex can have very different body water percentages depending on their body composition. A lean, muscular person will be closer to the top of the range, while someone carrying more fat tissue will fall toward the bottom.
Fitness level, hydration status, altitude, and even the temperature of your environment all shift the number on any given day. Medications like diuretics can lower total body water, while conditions that cause fluid retention can temporarily raise it. The “60% water” figure is a useful approximation, but your actual percentage is a moving target that reflects your unique body and circumstances.