Water in the human body
Roughly 55 to 60% of an adult is water, but the figure varies more between individuals than most people expect — and body composition matters more than body size.
The commonly quoted figure is "about 60% water". That is roughly right for an average adult man and noticeably wrong for many other people, because the number depends on body composition rather than on body size.
Why the proportion varies
Muscle tissue is roughly 75% water. Adipose tissue is roughly 10%. A person with more muscle and less fat therefore has a higher body water percentage than a person of identical weight with the opposite composition. Since men on average carry proportionally more muscle and less fat than women, the population averages differ — but the underlying driver is composition, not sex.
| Group | Total body water (% of body mass) |
|---|---|
| Newborn infant | ~75% |
| Child | ~65% |
| Adult man | ~60% |
| Adult woman | ~55% |
| Older adult | ~50% |
| Adipose tissue | ~10% |
| Skeletal muscle | ~75% |
| Blood | ~83% |
| Brain and kidneys | ~80% |
| Bone | ~31% |
Population averages. Individual values vary substantially and clinical assessment uses measurement rather than these figures.
Where the water sits
About two thirds of body water is intracellular — inside cells, where metabolism happens. The remaining third is extracellular, split between interstitial fluid bathing the cells and blood plasma. Plasma is only about a quarter of extracellular fluid, so blood accounts for a modest share of total body water despite being the fluid people think of first.
How balance is maintained
| Route | Approximate volume (ml/day) |
|---|---|
| In — drinks | 1,500 |
| In — food | 700 |
| In — metabolic water from respiration | 300 |
| Out — urine | 1,500 |
| Out — insensible loss through skin | 450 |
| Out — breath | 350 |
| Out — faeces | 200 |
Indicative only. Sweat losses in heat or exercise can exceed a litre per hour and dominate the balance entirely.
Metabolic water is often overlooked: oxidising carbohydrates and fats produces water as a by-product, contributing a few hundred millilitres a day. It is why some desert animals can survive without drinking at all.
What the kidneys do
The kidneys filter roughly 180 litres of plasma a day and reabsorb almost all of it, excreting one to two litres as urine. The concentration of that urine is adjusted continuously by antidiuretic hormone in response to plasma osmolality. This is the mechanism by which the body regulates concentration rather than volume, and it is why a healthy person can tolerate a wide range of intakes.
Sources
- National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for Water and Electrolytes. Public but restricted · Publisher terms — public access, reuse not clearly granted
- European Food Safety Authority — EFSA scientific opinions on water-related substances. Open — attribution required · CC BY 4.0