Your body is roughly 60% water by weight, though that number shifts depending on your age, sex, and body composition. For a 150-pound person, that translates to about 11 gallons of water distributed across every organ, tissue, and cell. The percentage isn’t fixed: it can range from about 45% to 75% across the population, with lean, muscular people carrying more water than those with higher body fat.
Where All That Water Lives
Water isn’t evenly spread throughout your body. Your lungs hold the most at roughly 83% water, which makes sense given their role in gas exchange across moist membranes. Your brain and heart are each about 73% water. Muscles and kidneys sit around 79%. Even your bones, which feel completely solid, are about 31% water.
At the cellular level, about 62% of your total body water sits inside your cells (intracellular fluid), while the remaining 38% circulates outside them in blood plasma, the fluid between tissues, and lymph. This ratio stays remarkably stable in healthy adults, and your kidneys work constantly to maintain it. When the balance shifts, even slightly, you feel the effects through thirst, fatigue, or changes in blood pressure.
Why Body Composition Changes the Number
The single biggest factor in your personal water percentage is the ratio of muscle to fat in your body. Muscle tissue is about 79% water, while fat tissue holds considerably less. This is why two people who weigh exactly the same can have very different total body water levels. A muscular person might be 65% water while someone with a higher body fat percentage might be closer to 50%.
This also explains the sex-based difference that shows up in averages. Adult men tend to carry more water as a percentage of body weight (around 60%) compared to adult women (around 55%), largely because men on average carry more lean muscle mass. The gap narrows when you compare men and women with similar body compositions.
Age plays a role too. Newborns are roughly 75% water, which gradually decreases through childhood and adolescence. Older adults tend to have lower percentages, partly because muscle mass naturally declines with age and partly because the body’s ability to regulate fluid balance becomes less efficient.
How Much Water You Need Daily
Your body loses water constantly through breathing, sweating, and urination, so you need a steady intake to maintain that 60% balance. General guidelines suggest about 15.5 cups (3.7 liters) of total fluid daily for men and 11.5 cups (2.7 liters) for women. “Total fluid” includes water from food, which typically accounts for about 20% of your daily intake. Fruits, vegetables, soups, and even coffee all contribute.
These numbers are averages for healthy adults in temperate climates. Your actual needs go up with exercise, heat exposure, illness (especially fever or vomiting), and pregnancy or breastfeeding. The simplest check is your urine color: pale yellow means you’re well hydrated, while dark yellow signals you need more fluids.
What Happens When Water Levels Drop
Losing just 1% to 2% of your body weight in water is enough to trigger noticeable symptoms: thirst, reduced concentration, and a dip in physical performance. For a 160-pound person, that’s only 1.5 to 3 pounds of water loss, which can happen easily during a hard workout or a hot day without enough fluids.
At 3% to 5% loss, symptoms escalate to headaches, dizziness, and significant fatigue. Beyond that, dehydration becomes a medical concern, with risks including rapid heart rate, confusion, and in extreme cases, organ damage. Your body has powerful mechanisms to prevent this, primarily thirst and the kidneys’ ability to concentrate urine and retain water, but those mechanisms can be overwhelmed during intense exercise, illness, or in older adults whose thirst signals weaken.
Measuring Your Body Water
If you’ve ever stepped on a smart scale that reports body water percentage, it used a method called bioelectrical impedance analysis. The scale sends a tiny electrical current through your body and measures how quickly it travels. Water conducts electricity well, so the speed of the signal estimates how much water you’re carrying.
These devices are convenient but not perfectly accurate. Research comparing smart scale readings against gold-standard laboratory measurements found that the scales tend to overestimate total body water by about 1 to 1.5 liters. They’re useful for tracking trends over time, like noticing a gradual change across weeks, but any single reading can be off by a meaningful amount. Hydration status, recent meals, and even skin temperature can all shift the result.
The gold standard for measuring total body water involves drinking a small amount of specially labeled water and tracking how it dilutes in your body over several hours. This method is precise but only used in research settings, so for practical purposes, a smart scale combined with attention to thirst and urine color gives most people a reasonable picture of their hydration.