The human body is approximately 50–60% water by total weight — 60% in adult males and 50% in adult females on average. A 70 kg man carries roughly 42 litres of water distributed across cells, blood, organs, and interstitial fluid. Water percentage varies significantly by organ, age, and body composition, with the brain and kidneys containing the highest concentrations (approximately 80%) and bone and adipose tissue the lowest (10–30%). This guide covers the distribution of water throughout the body, why it varies between individuals, and why maintaining this balance is essential to every biological function.
Water Percentage by Organ
Each organ contains a different proportion of water, reflecting its function and metabolic activity:
- Brain: 73–80% water. Supports neurotransmitter production, cerebrospinal fluid, and electrical signal transmission. Even mild dehydration (1–2% body water loss) measurably impairs cognitive function.
- Kidneys: 79–83% water. The highest water content of any organ, required for filtration of approximately 180 litres of blood daily.
- Heart: 73% water. Maintains blood volume and cardiac muscle function.
- Lungs: 83% water. Supports gas exchange and maintains the moist lining of the alveoli.
- Muscles: 75% water. Muscle tissue is approximately three-quarters water, which is why muscular individuals have higher total body water percentages.
- Skin: 64% water. Maintains elasticity, barrier function, and thermoregulation.
- Bones: 22–31% water. Lower than soft tissue but essential for bone marrow function and structural integrity.
- Adipose tissue (fat): 10–15% water. The lowest water content of any tissue type, which is why body fat percentage significantly affects total body water proportion.
How Body Water Percentage Varies by Age
Body water percentage decreases across the lifespan:
Newborns and infants contain approximately 75–78% water. This high proportion reflects the large ratio of lean tissue to fat and the rapid metabolic rate of growth. Infants are exceptionally vulnerable to dehydration because their high water content means that proportional losses represent a larger fraction of total body water.
Children (ages 1–10) contain approximately 65–70% water as body composition shifts and fat stores develop.
Adults (ages 20–50) stabilise at approximately 50–60%, with the variation primarily driven by sex and body composition differences.
Elderly adults (ages 65+) experience a decline to approximately 45–55% body water. This reduction is caused by loss of lean muscle mass (sarcopenia), increased relative fat proportion, and reduced kidney concentrating ability. The lower baseline water reserve in elderly adults is one reason why dehydration risk increases with age and why consistent daily water intake becomes more important even as thirst perception diminishes.
Why Men and Women Have Different Body Water Percentages
The 10-percentage-point difference between male (60%) and female (50%) body water content is almost entirely explained by body composition. Women typically carry a higher proportion of body fat (which is only 10–15% water) and lower proportion of muscle mass (which is 75% water) compared to men. The water content of equivalent tissues is identical between sexes — the difference lies in the ratio of tissue types.
This has practical implications for hydration: women reach equivalent dehydration levels at lower absolute water losses because their total water reserve is smaller. It also means that overhydration thresholds are lower for women — the same volume of excess water produces greater dilution of electrolytes in a smaller total water pool.
What Roles Water Performs in the Body
Water's functions in the body can be categorised into 6 essential roles:
Solvent and transport medium: Water dissolves nutrients, electrolytes, and waste products, carrying them between cells, organs, and excretory systems. Blood plasma (90% water) is the primary transport vehicle.
Temperature regulation: Water absorbs metabolic heat and distributes it to the skin surface, where evaporation through sweat dissipates excess heat. Without this mechanism, core body temperature would rise to fatal levels within hours of physical activity.
Structural component: Water provides shape and rigidity to cells through hydrostatic pressure (turgor). The spinal cord and brain are cushioned by cerebrospinal fluid (99% water).
Biochemical reactant: Water participates directly in metabolic reactions including hydrolysis (breaking down macronutrients) and hydration reactions (synthesising new molecules). Digestion is fundamentally dependent on water as a reactant.
Lubricant: Synovial fluid (water-based) lubricates joints. Mucus (water-based) protects and lubricates the digestive, respiratory, and urogenital tracts. Saliva (99% water) lubricates food for swallowing and initiates digestion.
Waste removal: The kidneys use water to dissolve and excrete metabolic waste (urea, creatinine, uric acid) through urine. Inadequate water reduces excretion efficiency, allowing waste to accumulate.
Every one of these functions degrades when water intake falls below the body's requirements. The full range of health benefits attributed to drinking water are direct consequences of maintaining these 6 functions at optimal levels.
Why Body Water Matters for Understanding the Water Crisis
The human body's dependence on water is not theoretical — it is structural. Every organ requires water to function. Every metabolic process consumes or depends on water. The 42 litres of water in a 70 kg adult must be continuously replenished because the body has no mechanism for storing surplus water.
For communities without access to clean water, this biological reality means that every day without adequate safe water is a day of compromised organ function, reduced immunity, and accumulated health damage. Children, with their higher percentage of body water and faster metabolic rates, are affected first and most severely.
How long a person survives without water — approximately 3 days — is determined by how quickly the body's water reserves are depleted below the threshold needed to sustain organ function. A hand water pump providing clean groundwater ensures that this threshold is never reached, day after day, for the entire community the pump serves.