The human body is roughly 50 to 75 percent water, with the average adult falling around 60 percent. That number shifts depending on your age, sex, and how much muscle versus fat you carry. A 150-pound person at 60 percent water content is holding about 11 gallons of water inside their body at any given moment.
Average Percentages by Sex
Adult men average about 60 percent water by body weight, while adult women average closer to 55 percent. The difference comes down to body composition. Fat tissue holds significantly less water than lean tissue. About 73 percent of lean body mass is water, while fat tissue is far more variable, ranging anywhere from 17 to 84 percent water depending on the type and location of fat. Since women naturally carry a higher proportion of body fat than men, their overall water percentage tends to be lower.
This also means two people of the same sex and age can have noticeably different body water percentages based on fitness level alone. A muscular person will have a higher water percentage than someone of the same weight with more body fat.
How Body Water Changes With Age
Newborns are the most water-dense humans. From birth to six months, babies average about 74 percent water, with a wide range of 64 to 84 percent. By the time a child reaches six months to a year old, that average drops to about 60 percent and stays relatively steady through childhood.
The decline picks up again later in life. Men over 51 average about 56 percent water, while women in the same age group average around 47 percent. That lower number in older women reflects both the natural increase in body fat with aging and the hormonal changes that shift body composition after menopause. The ranges are broad: older men can fall anywhere between 47 and 67 percent, and older women between 39 and 57 percent.
Where the Water Actually Sits
Not all of your body’s water is sloshing around freely. About 62 percent of total body water is stored inside your cells, in what physiologists call intracellular fluid. The remaining 38 percent sits outside cells as extracellular fluid, which includes blood plasma, the fluid between tissues, and the liquid surrounding your spinal cord and joints.
That 62-to-38 ratio is a useful marker of health. When it shifts, meaning more water moves outside cells relative to inside, it can signal problems like inflammation, malnutrition, or kidney dysfunction. In healthy people, the ratio stays remarkably stable across most of adulthood.
What All That Water Does
Water isn’t just filling space. It’s actively involved in keeping you alive in ways that go far beyond quenching thirst. It carries nutrients and oxygen to your cells through your bloodstream, which is itself about 90 percent water. It regulates your body temperature through sweating and the way blood flow distributes heat. It lubricates your joints, cushions your brain and spinal cord, and helps flush waste through your kidneys.
Even small drops in hydration, as little as 1 to 2 percent of body weight lost through fluid, can affect concentration, mood, and physical performance. Your body has no real way to store extra water for later use the way it stores energy as fat. It relies on a constant cycle of intake and output, which is why consistent fluid consumption matters more than occasional large amounts.
How Body Water Is Measured
If you’ve ever stepped on a smart scale that claims to measure your body water, it was likely using a method called bioelectrical impedance analysis, or BIA. This works by sending a tiny electrical current through your body. Water conducts electricity well, so the device estimates your total body water based on how easily the current passes through your tissues. It takes about five minutes and is painless.
The gold standard for accuracy is isotope dilution, where you drink a small amount of water tagged with a traceable marker and then provide blood or urine samples so researchers can calculate how much water your body contains by seeing how diluted the marker becomes. This method is primarily used in research settings. BIA has been validated against it in numerous studies and is considered a reasonable alternative for everyday clinical use, though individual readings can vary with hydration status, recent exercise, or even the time of day.
Why Individual Numbers Vary So Much
The 50 to 75 percent range published by the National Athletic Trainers’ Association is wide for good reason. Body water percentage is not a fixed number. It changes throughout the day as you eat, drink, sweat, and urinate. It changes across your lifespan, dropping from the mid-70s at birth to the upper 40s in some older adults. It changes with body composition, fitness level, and even altitude.
For most healthy adults, landing somewhere between 50 and 65 percent is normal. Rather than fixating on a precise number, the practical takeaway is that your body is, by a comfortable margin, more water than anything else, and maintaining that balance is one of the simplest and most important things you can do for your health.