The human body is about 60% water by weight in a healthy adult, though the actual number varies from roughly 45% to 75% depending on your age, sex, and body composition. The commonly quoted “70% water” figure isn’t wrong for everyone, but it overstates the average. For most adults, 60% is the more accurate benchmark.
Why the Number Isn’t the Same for Everyone
The single biggest factor that shifts your body’s water percentage is how much fat versus muscle you carry. Muscle tissue is about 79% water, while fat tissue holds considerably less. This means a lean, muscular person can be closer to 65% water, while someone with a higher body fat percentage might be closer to 50%.
This also explains the difference between sexes. Adult men typically fall in the range of 55% to 65%, while adult women average 45% to 55%. Women naturally carry a higher proportion of body fat relative to lean tissue, which lowers the overall water percentage. The gap has nothing to do with hydration habits; it’s purely a matter of tissue composition.
How Body Water Changes With Age
Newborns are the most water-dense humans. A baby’s body is roughly 75% water at birth, which gradually decreases through infancy and childhood. Children still carry a higher proportion than adults, typically 60% to 70% of their body weight as water.
By adulthood, the number settles around 60% and holds relatively steady for decades. Then in older age, it starts to decline again. As people age, they tend to lose muscle mass and gain fat tissue, both of which push the water percentage downward. An older adult over 70 may carry noticeably less total body water than they did at 30, even at the same weight. This shift is one reason older adults are more vulnerable to dehydration.
Where All That Water Actually Sits
Your body’s water isn’t sloshing around in one big pool. It’s divided into two main compartments in roughly a 2:1 ratio. About two-thirds of your total body water sits inside your cells, known as intracellular fluid. This is the water that fills every cell in your muscles, organs, and tissues, making up the environment where your cells do their work.
The remaining one-third is extracellular fluid: the water outside your cells. This includes blood plasma, the fluid between your tissues, and specialized fluids like cerebrospinal fluid and the liquid inside your joints. Blood plasma alone is about 90% water, which is what makes blood effective at carrying nutrients, oxygen, and waste products throughout your body.
Water Content by Organ
Not all parts of your body hold water equally. Your muscles and kidneys are among the most water-rich, at about 79%. Your brain, heart, and lungs are similarly high, generally in the 70% to 80% range. Even your bones contain water, though at a much lower percentage.
Fat tissue sits at the low end, which is why body composition matters so much for your overall percentage. If you replaced five pounds of fat with five pounds of muscle, your total body water percentage would rise, even though you weigh the same.
What All This Water Does
Water isn’t just filling space. It’s the medium in which nearly every chemical reaction in your body takes place. Your cells rely on water to break down nutrients, build proteins, and generate energy. Water carries dissolved oxygen and glucose to tissues through your bloodstream and flushes waste products out through your kidneys.
It also plays a critical role in temperature regulation. When you overheat, your body pushes water to the skin’s surface as sweat, and evaporation cools you down. Water cushions your brain and spinal cord, lubricates your joints, and keeps mucous membranes moist so they can function as barriers against infection. Losing even 2% of your body water is enough to impair concentration, increase fatigue, and reduce physical performance.
How Body Water Is Measured
If you’ve ever stepped on a “smart” scale at a gym or doctor’s office, it likely estimated your body water using a method called bioelectrical impedance analysis, or BIA. The device sends a tiny, painless electrical current through your body and measures how easily it passes. Water and electrolytes conduct electricity well, so the more resistance your body offers, the less water (and more fat) it contains.
BIA is portable, inexpensive, and noninvasive. When combined with your height and weight, it can predict total body water with an accuracy within about 1.75 liters of the gold-standard laboratory method. That lab method involves drinking a small amount of labeled water (containing a traceable form of hydrogen) and measuring how it dilutes in your body over several hours. It’s highly precise but impractical for everyday use, which is why BIA dominates in clinical and fitness settings.
How Much Water You Need Daily
Maintaining that 60% requires a constant cycle of intake and loss. You lose water through urine, sweat, breathing, and digestion every day. Current guidelines suggest that the average healthy adult needs about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid daily, with the higher end applying to men. That total includes water from all sources: plain water, other beverages, and the moisture in food, which typically accounts for about 20% of your daily intake.
Your actual needs shift with activity level, climate, illness, and pregnancy or breastfeeding. Thirst is a reasonable guide for most healthy people, though it becomes less reliable with age, which circles back to why older adults need to be more deliberate about drinking fluids regularly.