Water makes up about one half to two thirds of your body weight, depending on your age, sex, and body composition. For the average adult man, that figure sits around 60%. For the average adult woman, it’s closer to 52 to 55%. The difference comes down mostly to body fat, which holds far less water than muscle.
Why the Percentage Varies So Much
The range between 50% and 65% is wide, and that’s because no two bodies are built the same way. The single biggest factor is how much of your body is lean tissue versus fat. Lean tissue (muscle, organs, connective tissue) is roughly 70% water by weight. Adipose tissue, or body fat, is only about 20% water. That’s a massive gap: lean tissue holds approximately six times more water per unit of weight than fat does.
This explains the sex-based difference. Women, on average, carry a higher percentage of body fat than men, which means a lower overall water percentage. It also explains why two men of the same height and weight can have different body water levels if one is more muscular and the other carries more fat.
How Body Water Changes With Age
Newborns are the most water-dense humans. Infants are around 70 to 80% water, partly because of their higher metabolic rate and a larger proportion of extracellular fluid that supports rapid growth. As children grow into young adults, that percentage gradually drops into the 55 to 60% range.
The decline continues into older age. Elderly adults typically have the lowest body water content of any age group. The reasons are straightforward: aging brings increased fat deposits and decreased muscle mass, both of which shift the balance away from water-rich tissue. This is one reason older adults are more vulnerable to dehydration. Their water reserves are simply smaller to begin with.
Where the Water Actually Sits
Your body water isn’t sloshing around freely. It’s divided into two main compartments in roughly a 2:1 ratio. About two thirds of your total body water is intracellular fluid, meaning it’s inside your cells, distributed across every tissue in your body. The remaining third is extracellular fluid, which includes blood plasma, the fluid between cells, and fluid in places like the spinal canal and eyes.
For a 154-pound (70 kg) man at 60% water, that’s about 42 liters of total body water. Around 28 liters would be inside cells, and 14 liters outside them. This distribution matters because your body works constantly to keep the balance right. When the ratio shifts, even slightly, it triggers thirst, changes in blood pressure, and signals to your kidneys to retain or release water.
Water Content of Individual Organs
Some parts of your body are far more water-rich than others. Your skin, which you might not think of as a particularly watery organ, is 64% water. Even your bones, the hardest structures in your body, are 31% water. At the other end of the spectrum, organs with high metabolic demands tend to be the most hydrated. The brain, heart, lungs, kidneys, and liver are all well above 70% water, consistent with the general finding that lean, metabolically active tissue hovers around 70% hydration.
Blood is one of the most water-rich substances in the body, which makes sense given its job as a transport system. Teeth are at the very bottom of the scale, with almost no water content.
What All That Water Does
Water isn’t just filling space. It’s an active participant in nearly every function your body performs. It regulates your body temperature through sweating and blood flow to the skin. It lubricates your joints, reducing friction between cartilage surfaces. It cushions and protects your brain, spinal cord, and other organs. It carries nutrients and oxygen to cells and flushes waste products through your kidneys and liver.
Water also acts as a solvent, dissolving minerals, vitamins, and other nutrients so your body can actually absorb and use them. Without adequate water, these processes start to break down in a predictable order: first you feel thirsty and your urine darkens, then cognitive performance drops, and eventually organ function is affected.
How Much Water You Lose Each Day
On a typical day, your body loses about 2,500 milliliters (roughly 2.6 quarts) of water through several routes. The largest share, about 1,500 milliliters, leaves as urine. The rest is split between what’s called insensible water loss, which is evaporation from your skin and moisture in the air you exhale, plus smaller amounts in stool.
You’re usually unaware of insensible losses because they happen continuously and invisibly. On a hot day or during exercise, sweat adds significantly to the total. To stay in balance, your body needs roughly the same 2,500 milliliters coming in each day, through a combination of drinking fluids and water contained in food. Fruits, vegetables, and cooked grains all contribute meaningful amounts of water to your daily intake.
Your kidneys are the primary regulators of this balance. When you’re well hydrated, they produce more dilute urine to shed the excess. When you’re low on water, they concentrate your urine to conserve fluid. This system is remarkably precise in healthy adults but becomes less responsive with age, which is another reason dehydration risk climbs in older populations.