The human body is roughly 60% water by weight. For a 154-pound person, that translates to about 11 gallons of water distributed across every organ, tissue, and cell. The exact percentage varies depending on your age, sex, and body composition, but water is by far the most abundant substance in your body.
How the Percentage Changes With Age and Sex
Newborns and young children carry the highest proportion of water, with it making up 60% to 70% of their total body weight. Adults generally fall in the 60% range, though men tend to have a slightly higher water percentage than women because they typically carry more muscle mass (muscle holds far more water than fat). As people age and naturally lose muscle while gaining body fat, total body water gradually drops below that 60% mark.
The key variable is body composition. Lean tissue is approximately 73% water, while fat tissue is far drier and more variable, ranging anywhere from 17% to 84% water depending on the type and location. This is why two people of the same weight can have meaningfully different total body water: the person with more muscle is, quite literally, more water.
Where All That Water Actually Sits
About two-thirds of your body water is inside your cells, a compartment physiologists call intracellular fluid. The remaining third sits outside cells. Of that external portion, roughly 75% fills the spaces between cells in your tissues, while the other 25% is plasma circulating in your bloodstream. A small additional fraction (2% to 3%) exists in specialized pockets like the fluid surrounding your brain and spinal cord, your lymphatic system, and your eyes.
That distribution matters because your body constantly shuttles water between these compartments to maintain blood pressure, deliver nutrients, and keep cells functioning. When you become dehydrated, the balance shifts, and your body pulls water from less critical areas to protect blood volume.
Water Content of Individual Organs
Not every part of your body holds the same amount of water. Some organs are surprisingly water-dense:
- Lungs: 83%
- Muscles and kidneys: 79%
- Brain and heart: 73%
- Liver: 71%
- Bones: 31%
- Blood: 90% (plasma specifically)
Even your skeleton, which feels completely solid, is nearly one-third water. Your lungs top the list because the thin, moist membranes that allow oxygen exchange require constant hydration to function. Blood plasma, at 90% water, serves as the liquid highway that carries proteins, nutrients, and immune cells throughout your body.
What All That Water Does
Water isn’t just filling space. It plays active roles in nearly every biological process.
Your body produces heat constantly, especially during physical activity, and water is the primary cooling system. Sweat carries heat to the skin surface, where evaporation pulls that heat away. During exercise, sweat rates typically range from 0.5 to 2.0 liters per hour, and some athletes produce over 3 liters per hour in hot conditions. Without adequate water to fuel this process, core body temperature rises dangerously fast.
Water also keeps your joints moving smoothly. Cartilage is 65% to 80% water, and the synovial fluid that cushions joints like your knees, hips, and shoulders depends on hydration to act as a shock absorber. When you’re well-hydrated, that fluid reduces friction and prevents bones from grinding against each other during movement.
At the cellular level, water serves as the solvent for virtually every chemical reaction in your body. It transports oxygen to tissues, flushes waste products through your kidneys, and helps maintain the electrical balance that lets your nerves fire and your muscles contract.
How Much Water You Lose Each Day
Your body sheds water continuously, even when you’re sitting still. In an average day for a sedentary adult, the losses break down roughly like this:
- Urine: 1,500 ml
- Insensible loss through skin and breathing: 900 ml
- Feces: 100 ml
- Sweat (at rest): 50 ml
That adds up to about 2,550 ml, or roughly 10.5 cups, lost daily before you factor in exercise or hot weather. Breathing alone accounts for a surprising share: every exhale carries water vapor out of your lungs.
To replace those losses, healthy adults generally need 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day from all sources, including food. Fruits, vegetables, soups, and other foods with high water content contribute a meaningful portion of that intake.
What Happens When Water Levels Drop
Because your body is so water-dependent, even small deficits have measurable effects. Losing just 2% of your body weight in fluid (about 3 pounds for a 150-pound person) is enough to impair both physical performance and cognitive function. Reaction times slow, endurance drops, and concentration becomes harder to sustain. Greater losses compound those effects and can become medically dangerous.
Your body has a built-in early warning system: thirst. But thirst typically kicks in after you’ve already lost 1% to 2% of body weight in water, which means by the time you feel thirsty, your performance is already starting to decline. The color of your urine is a more reliable real-time gauge. Pale yellow generally signals adequate hydration, while dark yellow or amber suggests you need more fluid.