The average adult human body is about 50 to 60% water by weight. For a 70-kilogram (154-pound) man, that works out to roughly 38 to 45 liters, or about 10 to 12 gallons. The exact percentage depends on your age, sex, and how much muscle versus fat you carry.
Water Content by Age and Sex
Babies are the most water-dense humans. From birth to six months, an infant’s body averages 74% water, with a range of 64 to 84%. That percentage drops steadily over a lifetime as body composition shifts.
Adult men between 19 and 50 average about 59% water (range: 52 to 66%), while adult women in the same age group average about 50% (range: 41 to 60%). The gap comes down to body fat. Women naturally carry a higher proportion of fat tissue, which holds far less water than muscle. After age 50, those numbers dip a few more points: men average 56% and women average 47%.
Why Muscle and Fat Make Such a Difference
Lean tissue is roughly 73% water. Fat tissue is much drier, and its water content is highly variable, ranging anywhere from about 17 to 84% depending on the type of fat deposit. This is the single biggest reason two people of the same weight can have very different total body water. A muscular person carries substantially more water than someone with a higher body fat percentage, even at identical scale readings. It also explains why men, who on average have more lean mass, tend to run a higher water percentage than women.
Where the Water Sits Inside You
Not all of your body water sloshes around freely. About two-thirds of it, roughly 40% of your total body weight, sits inside your cells. This is called intracellular fluid, and it’s where most of the chemistry of life happens. The remaining one-third, about 20% of body weight, lives outside your cells. This extracellular fluid includes the liquid portion of your blood (plasma), the fluid surrounding your cells in tissues, and smaller reservoirs like the fluid in your joints and eyes.
Blood itself is about 80% water overall. Plasma, the yellowish liquid that carries red and white blood cells, is 90% water. Your blood volume is only a fraction of your total body water, but it’s the compartment your body monitors most aggressively, adjusting kidney output and thirst signals to keep it stable.
Water Content of Individual Organs
Some organs are surprisingly water-rich. According to the U.S. Geological Survey, your lungs top the list at about 83% water. Your kidneys come in at 79%. The brain and heart are each about 73% water. Even your bones, which feel rock-solid, are roughly 31% water.
These numbers help explain why dehydration affects so many body systems at once. When total body water drops even a small amount, every organ from your brain to your kidneys is working with less of its primary building material.
How Much Water You Lose Each Day
Your body cycles through a significant volume of water every 24 hours. The kidneys account for the largest outflow, producing 1 to 2 liters of urine daily. Beyond that, you lose about 450 milliliters through your skin as invisible perspiration (not counting visible sweat during exercise or heat), 250 to 350 milliliters through the moisture in your breath, and about 200 milliliters through your digestive tract in stool. In a temperate climate without heavy exertion, that adds up to roughly 2 to 3 liters of water lost per day before you factor in sweat from physical activity or hot weather.
All of that water needs to be replaced through food and drink. Solid foods contribute more than most people realize, typically supplying 20 to 30% of daily water intake. The rest comes from beverages. Your body is remarkably good at matching intake to output through thirst, but the system isn’t perfect, particularly in older adults whose thirst signals tend to weaken with age.
Putting It in Perspective
For a quick estimate of your own total body water, multiply your weight in kilograms by 0.6 if you’re a younger adult male, or by 0.5 if you’re a younger adult female. A 70 kg man lands at about 42 liters. A 60 kg woman comes out to about 30 liters. These are rough averages, and your actual number could be higher or lower depending on your fitness level and body composition. The leaner you are, the closer you’ll be to the upper end of the range.