The average adult human body contains roughly 42 liters of water, based on a standard 70 kg (154 lb) person. That works out to about 60% of total body weight, though the exact number varies considerably depending on your age, sex, body size, and how much muscle versus fat you carry.
How Body Size and Composition Change the Number
The 42-liter figure is a useful benchmark, but it assumes a specific body type. A smaller person will carry less total water, and a larger person more. What really shifts the number, though, is body composition. Lean muscle tissue is about 70% water by weight, while fat tissue holds only about 14 to 20% water. This means two people who weigh the same can have meaningfully different amounts of total body water depending on their ratio of muscle to fat.
This is also why men typically have a higher percentage of body water than women. Men tend to carry more muscle mass and less body fat, so their total body water often lands around 60% of body weight. For women, the average is closer to 50 to 55%, largely because of naturally higher body fat percentages. A 70 kg woman might carry closer to 35 to 38 liters of water rather than 42.
Medical professionals sometimes use estimation formulas to calculate total body water more precisely. The Watson formula, one of the most widely used, factors in age, height, and weight for men, while using height and weight for women. For men, age pulls the number down slightly with each passing year, reflecting changes in body composition over time.
How Water Volume Changes With Age
Newborns are the most water-rich humans. Infants carry about 70 to 80% of their body weight as water, driven by a higher proportion of extracellular fluid and relatively little body fat. As children grow and accumulate more fat tissue, that percentage gradually drops.
Young adults settle into the 50 to 60% range, where most people stay through middle age. In older adults, total body water decreases further. Aging brings a natural loss of muscle mass and a relative increase in fat tissue, both of which reduce the body’s water stores. Elderly adults typically have the lowest body water percentage of any age group, which is one reason dehydration becomes a more serious risk later in life.
Where the Water Actually Sits
Not all 42 liters are doing the same job. Your body divides its water into two main compartments. About two-thirds, roughly 28 liters, sits inside your cells. This intracellular fluid is where most of the biochemical work of life happens: energy production, protein building, waste processing.
The remaining third, about 14 liters, is extracellular fluid, meaning it exists outside of cells. This includes the fluid between your cells (called interstitial fluid), blood plasma, and smaller reservoirs like cerebrospinal fluid and the fluid in your eyes and joints. Blood plasma alone accounts for a significant share. The average adult has about 5 liters of blood, and over 90% of the plasma portion is water, contributing roughly 2.5 to 3 liters to the extracellular total.
Water Content by Organ
Some organs are far more water-dense than others, and the differences are striking. Your lungs are the most water-rich major organ at about 83% water by weight. The brain and heart are each about 73% water. Muscles and kidneys come in at 79%, and your skin, the body’s largest organ, is roughly 64% water. Even bones, which feel completely solid, are about 31% water.
These percentages explain why dehydration affects certain functions before others. The brain’s high water content is part of why even mild fluid loss can cause headaches, difficulty concentrating, and fatigue well before you notice other physical symptoms.
Practical Ranges by Body Type
Because the “42 liters” figure assumes a specific body, here’s a rough sense of how the number shifts across different people:
- Small adult woman (50 kg): roughly 25 to 28 liters
- Average adult woman (65 kg): roughly 32 to 36 liters
- Average adult man (70 kg): roughly 38 to 42 liters
- Larger adult man (90 kg): roughly 45 to 50 liters
- Newborn infant (3.5 kg): roughly 2.5 to 2.8 liters
Athletes and people with higher-than-average muscle mass will trend toward the upper end of these ranges. People with higher body fat percentages will trend lower, even at the same weight. This is a normal variation, not a health concern on its own, but it does mean that blanket recommendations about daily water intake don’t apply equally to everyone. Your body’s actual water reservoir depends on the body you’re carrying it in.