The human body is roughly 50% to 65% water by weight, though the exact number depends on your age, sex, and how much body fat you carry. A lean adult man typically sits around 60%, while an adult woman averages closer to 55%. These aren’t fixed numbers. They shift throughout your life and even throughout the day.
Why the Range Varies So Much
The single biggest factor driving your personal water percentage is body composition. Lean tissue (muscle, organs, bone) is about 73% water, while fat tissue holds very little, roughly 5% to 10%. That gap explains nearly every variation you’ll see between individuals. A muscular person carries proportionally more water than someone with a higher body fat percentage, regardless of sex.
This is also why men tend to have a higher overall water percentage than women. Men on average carry more lean mass and less body fat. But the difference isn’t really about sex. It’s about fat. When researchers compare men and women with similar body compositions, cell hydration levels are virtually identical. The ratio of total body water to fat-free mass stays consistent across sex and age. So if you’re wondering where you personally fall on the 50% to 65% spectrum, your body fat percentage is the best predictor.
Newborns Start Much Higher
A full-term newborn is about 75% water. Premature babies born between 26 and 31 weeks of gestation can be as high as 80% to 85% water by weight. Much of this extra water sits outside the cells, in the fluid surrounding tissues. At 26 weeks of gestation, that extracellular water accounts for roughly 65% of body weight. By the time a baby is born at full term, it drops to about 40%. By age 10, it settles to around 20%, which is close to adult levels.
This rapid decline happens because the body gradually replaces fluid-heavy tissue with denser structures like muscle, bone, and fat during growth. It’s one of the reasons infants are so vulnerable to dehydration: they have more water to lose, and they lose it faster relative to their size.
Water Content by Organ and Tissue
Not every part of your body holds water equally. Blood is one of the most water-rich substances in the body. Plasma, the liquid portion that makes up most of your blood volume, is about 90% water. It serves as the transport highway for nutrients, oxygen, hormones, and waste products.
Lean tissues like the brain, heart, lungs, and liver all contain significantly more water than fat or bone. Skeletal muscle is especially water-dense because of its high metabolic activity and rich blood supply. Fat tissue, on the other hand, acts almost like a dry reservoir. Its low water content (sometimes as little as 5% to 10%) is why gaining body fat dilutes your overall body water percentage even if your actual hydration hasn’t changed.
What All That Water Actually Does
Water isn’t just filling space. It performs specific jobs in nearly every system of the body. It carries nutrients and oxygen to cells, flushes waste through the kidneys and liver, lubricates joints, and dissolves minerals so the body can absorb them. It also regulates temperature: when you overheat, water moves to the skin’s surface as sweat and cools you through evaporation. Even the moisture in your eyes, nose, and mouth depends on a steady supply of body water to keep tissues functional.
Your body is constantly cycling through this supply. On a normal day, you lose about 1.5 pints (just under a liter) of water through evaporation from the skin and breathing alone. Urine accounts for a much larger and more variable share. Depending on your hydration, activity level, and diet, the kidneys may produce as little as half a liter or more than 10 liters of urine per day. Severe diarrhea or prolonged vomiting can push total daily water loss above a gallon, which is why those conditions cause dehydration so quickly.
How Body Fat Changes the Math
If you’ve ever seen dramatically different water percentages quoted online (anywhere from 45% to 75% for adults), body fat is almost always the explanation. A person with obesity might fall below 50% total body water, not because they’re dehydrated, but because a larger proportion of their body mass is adipose tissue, which contributes very little water. An elite endurance athlete with low body fat might exceed 60%. Both can be perfectly well-hydrated.
This also means that BMI-related changes in body composition shift your water percentage over time. Weight gain from fat lowers it. Gaining muscle raises it. Aging tends to push the number down as well, since older adults typically lose lean mass and gain fat, even if their weight stays the same. The 50% to 65% range is a useful benchmark, but your personal number is shaped by the composition underneath.