Water makes up about one half to two thirds of your total body weight. For an average 154-pound man, that translates to roughly 10.5 gallons (42 liters) of water. The exact percentage depends on your age, sex, and body composition, but for most adults it falls between 50% and 60%.
How Body Water Differs by Sex and Body Fat
The average adult man carries about 60% of his body weight as water, while the average woman carries 52% to 55%. The difference comes down to body composition. Fat tissue holds much less water than muscle, and women on average have a higher proportion of body fat relative to lean mass. This means two people who weigh the same can have meaningfully different amounts of total body water depending on how much of that weight is muscle versus fat.
This also explains why athletes and people with more muscle mass tend to have a higher water percentage, while someone with more body fat will land on the lower end of the range regardless of sex.
Changes Across Your Lifespan
Newborns are the most water-dense humans, with water making up roughly 75% of their body weight. That percentage gradually declines through childhood and into adulthood. Older adults tend to have the lowest body water percentage because aging brings both a natural loss of muscle mass and an increase in fat tissue. This is one reason older adults are more vulnerable to dehydration: they’re starting from a smaller reserve.
Even in a healthy, weight-stable adult, total body water fluctuates by about 5% on any given day. Eating, drinking, sweating, and urinating all shift the balance hour by hour.
Where the Water Actually Sits
Your body’s water isn’t sloshing around in one big pool. It’s divided into distinct compartments. About two thirds of it (roughly 28 liters in a 154-pound man) is inside your cells, where it serves as the medium for virtually every chemical reaction that keeps you alive. The remaining third is outside your cells: about 10.5 liters in the spaces between cells and tissues, and roughly 3.5 liters (about 8% of total body water) circulating in your blood.
These compartments are carefully maintained. Your body constantly adjusts the balance of water and dissolved minerals between them. When that balance breaks down, whether from illness, medication, or extreme conditions, the effects can range from mild swelling to serious organ stress.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. Blood is the most water-rich tissue at about 90%. Muscles come in at roughly 79%, and the brain sits around 73%. Even your bones, which feel completely solid, are about 31% water. Fat tissue is among the driest in the body, which is why body fat percentage is the single biggest factor in determining your overall water content.
What All That Water Does
Water isn’t just filling space. It performs specific jobs in nearly every system of your body. It regulates your temperature through sweating and blood flow to the skin. It carries nutrients and oxygen to your cells and flushes waste products through the kidneys and liver. It lubricates your joints and cushions your organs, and it keeps the tissues of your eyes, nose, and mouth moist. It also dissolves minerals and other nutrients so your body can actually absorb and use them.
Losing even a small percentage of your body water, around 1% to 2%, is enough to impair concentration, increase fatigue, and reduce physical performance. By the time you feel thirsty, you’re typically already mildly dehydrated.
How Your Body Loses and Replaces Water
A healthy adult loses about 2,500 milliliters (roughly 10.5 cups) of water per day. The largest share leaves as urine, averaging about 1,500 milliliters daily. The rest exits through breathing, sweating, and digestion. Your kidneys need a minimum of about 500 milliliters of urine output per day just to clear waste products from the blood, so there’s a floor below which your body can’t reduce water loss without consequences.
You replace that water through drinking, eating (many foods are surprisingly water-dense), and a small amount generated internally by your metabolism. The system works on a tight feedback loop: sensors in your brain and kidneys detect shifts in water concentration and trigger thirst or adjust how much water your kidneys retain.
Conditions That Disrupt Water Balance
Several common medical situations can push your body’s water levels out of their normal range. Heart failure causes fluid to accumulate in the lungs, liver, and tissues because the heart can’t pump efficiently enough to move it through the kidneys. Chronic kidney disease reduces the kidneys’ ability to filter and excrete excess fluid. Liver disease can lower levels of a blood protein that normally keeps fluid inside blood vessels, causing it to leak into surrounding tissue and produce visible swelling.
On the other end, your body can lose water too quickly through prolonged diarrhea, vomiting, high fever, or severe blood loss. A deficiency in a hormone that tells the kidneys to hold onto water can also cause excessive fluid loss, leading to extreme thirst and rapid dehydration. Post-surgical patients often retain extra fluid for several days as the body’s stress response shifts into a temporary water-hoarding mode.