Water makes up roughly 50% to 70% of your total body weight, with the often-cited average for adults landing around 60%. That single number, though, hides a lot of variation. Your actual percentage depends on your age, sex, and how much muscle versus fat you carry.
How Age and Sex Shift the Number
Newborns are the most water-dense humans. An infant’s body is about 75% to 78% water, which drops to around 65% by their first birthday. From there, the percentage gradually declines through adulthood and into old age, when total body water can dip to 50% or even slightly below.
For adults, the split between men and women is consistent and well-documented. The average adult man is about 60% water by weight. The average adult woman is closer to 52% to 55%. The reason comes down to body composition: fat tissue holds far less water than lean tissue. Muscle and other fat-free tissue contain roughly six times more water per unit of weight than fat does. Since women on average carry a higher proportion of body fat, their overall water percentage is lower. This same principle explains why two people of the same sex and age can have meaningfully different body water percentages if one is leaner and the other carries more fat.
Where All That Water Actually Sits
Your body’s water isn’t evenly spread. About two-thirds of it is stored inside your cells, forming the fluid where most of your body’s chemical reactions take place. The remaining third sits outside cells: in your blood, in the fluid surrounding tissues, and in smaller reservoirs like spinal fluid and the moisture lining your digestive tract.
Different organs hold strikingly different amounts of water. Your lungs are the most water-rich major organ at about 83%. The brain and heart are each around 73%. Muscles and kidneys come in at roughly 79%. Even your skeleton contains water, though far less than soft tissue. Bone is denser and drier, and tooth enamel is the driest structure in the body. The pattern is intuitive once you think about it: the organs that do the most active metabolic work tend to need, and hold, the most water.
Why Body Composition Matters More Than Weight
If you’ve ever wondered why athletes are told they’re “well-hydrated” at a higher percentage than the general population, this is why. A muscular person carrying relatively little fat will have a total body water percentage on the higher end of the range, potentially above 60%, simply because muscle tissue is about 79% water. Fat tissue, by contrast, is only about 14% water. So two people who weigh exactly the same can differ by 10 or more percentage points in body water depending on their ratio of muscle to fat.
This also explains the age-related decline. As people get older, they tend to lose muscle mass and gain fat, which gradually shifts the overall water percentage downward. It’s not that older adults are necessarily dehydrated. Their body composition has simply changed in a way that holds less water per pound.
How Much Water You Cycle Through Daily
Your body doesn’t just store water; it constantly loses and replaces it. Even at rest, with minimal sweating and moderate temperatures, an adult loses roughly 1,600 milliliters (about 6.5 cups) of water per day through unavoidable routes. That breaks down to about 800 milliliters through breathing and evaporation from the skin, 500 milliliters as the minimum urine your kidneys need to flush waste, 200 milliliters in stool, and around 100 milliliters in baseline sweat. Physical activity, heat, illness, or high altitude push those numbers considerably higher.
All of that water needs to be replaced through a combination of drinking fluids and eating water-rich foods. Fruits, vegetables, soups, and even cooked grains contribute meaningful amounts. Your body is constantly balancing intake against loss, adjusting urine concentration and triggering thirst to keep total body water within a narrow range. A drop of just 1% to 2% below your normal level is enough to trigger noticeable thirst and mild symptoms like fatigue or reduced concentration.
What a “Normal” Body Water Percentage Looks Like
There’s no single number that qualifies as ideal for everyone. A healthy range for an adult man is roughly 55% to 65%, and for an adult woman, roughly 50% to 60%. Smart scales and bioimpedance devices can estimate your body water percentage, though their accuracy varies depending on hydration status, recent meals, and the device itself. They’re useful for tracking trends over time rather than pinpointing an exact figure on any given day.
The practical takeaway is that being “mostly water” isn’t just a fun fact. It reflects how central water is to nearly every process in your body, from regulating temperature and cushioning joints to transporting nutrients and flushing waste. The exact percentage that applies to you depends on your age, sex, and build, but for most healthy adults, somewhere around 55% to 60% of your body weight is water at any given moment.