The average adult human body is about 55% to 60% water by weight. For men, the figure sits around 60%, while for women it’s typically 52% to 55%. That difference comes down to body composition: fat tissue holds much less water than lean tissue, and women on average carry a higher proportion of body fat.
How the Percentage Changes With Age
You don’t stay the same percentage of water throughout your life. Newborns are the most water-dense humans, at roughly 70% to 80% water by body weight. That high ratio supports their rapid growth and the large volume of fluid circulating outside their cells. As children grow and gain more fat and bone mass, the percentage gradually drops. By adulthood it settles into the 55% to 60% range, and it continues to decline with age as older adults tend to lose lean muscle and gain proportionally more fat.
Water Content Varies by Organ
The 60% figure is a whole-body average. Individual organs and tissues differ dramatically. Your lungs are the most water-rich major organ at about 83%. The brain and heart each contain roughly 73% water. Skeletal muscles and kidneys come in at around 79%.
At the other end of the spectrum, fat tissue averages only about 15% water, with the rest being mostly stored fat. Bones are also relatively dry compared to soft tissue. This is exactly why body composition matters so much to the overall number: the more muscle you carry relative to fat, the higher your total body water percentage will be.
Where All That Water Actually Sits
Your body’s water isn’t sloshing around freely. It’s distributed across two main compartments. About two-thirds of your total body water is inside your cells, where it serves as the medium for virtually every chemical reaction keeping you alive. The remaining one-third sits outside your cells, in your blood plasma, the fluid between tissues, and specialized fluids like cerebrospinal fluid and the liquid inside your eyes and joints.
Clinicians sometimes call this the 60-40-20 rule: 60% of body weight is water, 40% of body weight is the water inside cells, and 20% is the water outside cells. Those proportions stay remarkably stable in a healthy person because your kidneys and hormones constantly fine-tune how much water you retain or excrete.
What Body Water Actually Does
Water isn’t just filling space. It’s the solvent your body uses to dissolve nutrients, electrolytes, and waste products so they can be transported through your bloodstream and filtered by your kidneys. Every cell relies on water to maintain its shape and internal pressure, and the chemical reactions of metabolism happen in water-based solutions inside those cells.
Water also plays a central role in temperature regulation. When you overheat, your body pushes warm blood toward the skin and produces sweat. As that sweat evaporates, it pulls heat away from your body. This cooling system depends entirely on having enough fluid available. Water additionally cushions your brain and spinal cord, lubricates your joints, and helps form saliva and digestive fluids that break down food.
What Shifts Your Personal Percentage
Several factors push your body water percentage higher or lower than the textbook average. Body fat is the biggest one. A lean, muscular person can be well above 60% water, while someone with a high body fat percentage may fall below 50%, because fat tissue holds so little water relative to muscle. This is the primary reason men and women differ on average, and why two people of the same sex and age can have noticeably different percentages.
Age is the second major factor, as described above. Hydration status also causes short-term fluctuations. After heavy exercise or a long stretch without drinking, your total body water dips. After rehydrating, it rebounds. These shifts are temporary but can be significant enough to affect how you feel and perform. Even a loss of 1% to 2% of body water can impair concentration and physical endurance.