The average adult human body is roughly 55% to 60% water by weight. That means a 150-pound person carries about 80 to 90 pounds of water at any given moment. The exact percentage depends on your sex, age, and how much muscle versus fat you carry.
The Basic Numbers
For adult men, total body water typically accounts for about 60% of body weight. For adult women, it’s closer to 52% to 55%. The difference comes down to body composition: fat tissue holds far less water than lean tissue, and women on average carry a higher proportion of body fat.
These are averages. A lean, muscular man might be closer to 65% water, while someone with a higher body fat percentage could fall below 50%. Two people who weigh the same can have meaningfully different total body water depending on their build.
How Age Changes the Equation
Babies are born with the highest water content of any stage in life. Infants are roughly 75% water, which is one reason they’re so vulnerable to dehydration from illness or heat. Children still run higher than adults, with water making up about 60% to 70% of their body weight as they grow.
From adulthood onward, the trend reverses. As people age, they tend to lose muscle mass and gain body fat, both of which drive the water percentage down. An older adult may fall well below 55%. This shift also helps explain why older adults are more susceptible to dehydration: they simply have less water reserve to start with, and the thirst signal becomes less reliable with age.
Why Muscle and Fat Matter So Much
The single biggest factor that determines your body’s water percentage, aside from age, is the ratio of lean tissue to fat tissue. Lean tissue (muscle, organs, skin) is about 70% water by weight. Adipose tissue, or body fat, is only about 14% to 20% water. That’s roughly a six-to-one difference in total water content per unit of weight.
This is why two people of the same height and weight can have very different hydration profiles. A person with more muscle is carrying substantially more water in absolute terms. It’s also why athletes tend to have higher body water percentages than sedentary people, and why gaining or losing significant amounts of muscle or fat shifts the ratio.
Where the Water Actually Sits
Not all of your body water is sloshing around freely. About two-thirds of it sits inside your cells, in a compartment scientists call intracellular water. The remaining third is extracellular, meaning it’s in your blood plasma, the fluid between your cells, and specialized fluids like cerebrospinal fluid and the fluid in your eyes and joints.
In healthy people, the ratio of extracellular water to total body water stays remarkably stable at around 0.38 (meaning about 38% of your total water is outside cells and 62% is inside). When that ratio shifts significantly, it can signal a medical issue like fluid retention or kidney problems.
Every organ and tissue holds water, but in very different amounts. The lungs, brain, and heart are among the most water-rich organs, each containing roughly 70% to 80% water. Even bones, which seem completely solid, contain a meaningful amount of water. Blood is about 90% water, which makes sense given its role as the body’s transport system.
What All That Water Does
Water isn’t just filling space. It performs a surprising number of active functions that keep you alive. Your body uses water to regulate internal temperature through sweating and breathing. When you overheat, sweat evaporating from your skin pulls heat away. When you exhale, water vapor carries heat out of your lungs.
Water is also the transport medium for nearly everything your body moves around. The carbohydrates and proteins you eat are broken down and carried to cells through water in the bloodstream. Waste products travel the same way, eventually getting filtered out by the kidneys and flushed through urination.
Beyond transport and temperature, water acts as a physical shock absorber. The cerebrospinal fluid cushioning your brain and spinal cord is mostly water, protecting those delicate structures from impact. During pregnancy, amniotic fluid serves the same protective role for the developing fetus. Water also lubricates your joints, reducing friction between bones during movement, and forms the basis of saliva, which starts the digestive process before food even reaches your stomach.
How Body Water Is Measured
If you’ve ever stepped on a “smart” scale that estimates your body water percentage, it was using a technique called bioelectrical impedance. The scale sends a tiny electrical current through your body and measures how easily it travels. Water conducts electricity well, so the signal moves faster through hydrated tissue (muscle) and slower through fat.
The gold standard for measuring body water in clinical research is isotope dilution. You drink a small amount of water labeled with a traceable form of hydrogen, wait for it to distribute evenly throughout your body, then provide a blood or urine sample. By measuring how diluted the tracer becomes, researchers can calculate your total body water with high precision. Bioelectrical impedance provides a comparable estimate for most people, though it tends to slightly overestimate total body water by about 1.5 to 2 liters compared to the isotope method.
Practical Implications of Body Water
Knowing that you’re mostly water puts a few everyday experiences into perspective. Rapid weight fluctuations of two to five pounds over a day or two are almost always water shifts, not fat gain or loss. A salty meal, a hard workout, hormonal changes during the menstrual cycle, and even air travel can all cause your body to temporarily hold or release water.
Dehydration, even at mild levels of 1% to 2% of body weight, affects cognitive performance, mood, and physical endurance. For a 160-pound person, that’s losing just 1.5 to 3 pounds of water, an amount easily lost through normal sweating during exercise or on a hot day. The effects are reversible once you rehydrate, but they’re a reminder that maintaining your body’s water balance is one of the most basic and impactful things you can do for how you feel and function.