Water makes up roughly 50% to 65% of your total body weight. For an average adult, that translates to about 35 to 45 liters of water distributed across every organ, tissue, and cell. The exact percentage depends on your age, sex, and body composition, with leaner individuals carrying a higher proportion of water than those with more body fat.
Why the Range Varies So Much
The 50% to 65% range exists because body water content is closely tied to how much lean tissue versus fat tissue you carry. Lean tissue (muscle, organs, bone) is approximately 70% to 80% water by weight. Fat tissue, by contrast, contains only about 14% water. That sixfold difference means someone with more muscle mass will have a significantly higher body water percentage than someone of the same weight with more body fat.
This is the main reason men tend to fall toward the higher end of the range. On average, men carry more muscle and less body fat than women, placing them closer to 60% to 65% water. Women typically fall closer to 50% to 55%. These are averages, though. A highly muscular woman will carry more body water than a sedentary man of the same weight.
How Body Water Changes With Age
Newborns are the most water-dense humans, at about 75% water by body weight. By the time a baby reaches one year old, that drops to around 60%. Throughout childhood, body water gradually decreases, reaching adult values around age 12.
After age 65, total body water falls again. Older adults lose muscle mass and tend to gain proportionally more fat tissue, both of which push the percentage down. The kidneys also become less efficient at conserving water with age, which is one reason dehydration is more common and more dangerous in older adults.
Water Content of Individual Organs
Not every part of your body holds water equally. Some organs are surprisingly water-rich:
- Lungs: 83% water, the highest of any major organ
- Muscles and kidneys: 79% water
- Brain and heart: 73% water
- Skin: 64% water
- Bones: 31% water, even though they feel completely solid
Blood plasma, the liquid portion of blood that carries red and white blood cells, is 92% water. This high water content is what allows plasma to flow freely and transport nutrients, hormones, and waste products throughout your body.
Where Water Sits Inside You
Your body water isn’t floating around freely. It’s divided into two main compartments that serve different functions. About two-thirds of your total body water sits inside your cells, where it acts as the medium for virtually every chemical reaction that keeps you alive. The remaining one-third sits outside your cells: in your blood vessels, in the fluid between cells, and in smaller reservoirs like cerebrospinal fluid and the fluid inside your eyes and joints.
For someone whose body is 60% water by weight, that works out to roughly 40% of body weight as water inside cells and 20% as water outside cells. Physiologists call this the 60-40-20 rule. Your body monitors the balance between these compartments very closely, because even small shifts can affect blood pressure, cell function, and nerve signaling.
What All That Water Actually Does
Water is the solvent your body runs on. Every nutrient you absorb from food dissolves in water before reaching your cells. Every waste product your kidneys filter travels through water to get eliminated. But water’s most underappreciated job is temperature regulation.
Even at rest, your body generates heat from basic cellular metabolism, your heartbeat, breathing, and digestion. To prevent overheating, your body relies on evaporative cooling through sweat. Evaporating just one gram of water from your skin dissipates about 0.6 kilocalories of heat. At a moderate sweat rate of one liter per hour during exercise, that removes roughly 600 kilocalories of heat, which is an enormous amount of cooling capacity.
The tradeoff is that sweating costs you water and electrolytes. Your kidneys compensate by adjusting how much water gets reabsorbed before becoming urine. When you’re dehydrated, a hormone signals the kidneys to pull more water back into the bloodstream, concentrating your urine. When you’re well-hydrated, the kidneys let more water pass through. This constant adjustment is how your body keeps blood volume stable enough for your heart and circulatory system to function properly.
Water also cushions your brain inside the skull, lubricates your joints, and forms the basis of saliva and digestive fluids. It’s involved in so many processes simultaneously that losing even 2% of your body water can noticeably impair concentration, physical performance, and mood.
Body Fat’s Outsized Effect on Your Percentage
If you’ve ever used a smart scale or had a body composition test that reported your water percentage, the number is largely a reflection of your lean mass to fat mass ratio. Someone at 10% body fat might measure above 65% body water, while someone at 35% body fat could fall below 50%, and both readings are perfectly normal for their composition.
This relationship also explains why body water percentage can shift with weight changes. Gaining muscle through strength training tends to increase your water percentage, while gaining fat decreases it. Crash dieting often causes a rapid initial drop in weight that’s mostly water loss from muscle glycogen stores, not actual fat loss, which is why the weight tends to return quickly once normal eating resumes.