FreshWater Watch archive
The percentage of water in the body by age and sex, from the studies
This address: freshwaterwatch.thewaterhub.org once held a 2015 community blog post on FreshWater Watch, Earthwatch's citizen science programme, which now runs at freshwaterwatch.org. This page is The Water Hub's own, with the figures read again from the studies and medical references.
The share of the body that is water is highest at birth and falls through life: about 75 to 80% in a newborn, about 65% by the first birthday, around 60% in adult men and 50 to 55% in adult women, and lower again in older adults (USGS; MSD Manual). Most of the difference between people is fat, which holds far less water than lean tissue, so the same person's share changes as their body composition does.
Body water by age band and sex
The clearest recent age-band figures come from a 2023 study that measured 545 people aged 3 to 98 by bioimpedance (Lu and others, Kidney Research and Clinical Practice, 2023). Boys and girls were alike at about 62% to 63% up to age 10; from adolescence the share for girls and women fell to 56%, then 54% in adults and 50% from 61, while for boys and men it held at about 62% to 63% before falling to 57% from 61. Overweight adults aged 21 to 60 measured 49% (men) and 41% (women). Bioimpedance is not the reference method, so the table sets these beside the general figures medical references give.
| Age band | Male | Female | Source |
|---|---|---|---|
| Birth | about 78% | about 78% | USGS |
| Birth | about 80% | about 80% | Lu 2023, citing Friis-Hansen 1951 |
| Age 1 | about 65% | about 65% | USGS |
| 3 to 10, normal weight | 63% | 62% | Lu 2023 |
| 11 to 20, normal weight | 63% | 56% | Lu 2023 |
| 21 to 60, normal weight | 62% | 54% | Lu 2023 |
| 21 to 60, overweight | 49% | 41% | Lu 2023 |
| 61 and over, normal weight | 57% | 50% | Lu 2023 |
| Adults, general figure | about 60% | 50% to 55% | USGS; MSD Manual |
Why women and older adults carry less water
Fat tissue is only 20% to 30% water, while about 72% of the fat-free body is water (Lu 2023), so a body with more fat has a smaller share of water. Women carry more fat on average than men, which is the main reason their figure is lower, and the share falls with age as body fat rises: the Fels Longitudinal Study, measuring 566 adults by deuterium dilution, found the ratio of body water to weight declined with age as a function of rising fatness, more so in men (Chumlea and others, 1999). Some older adults measure as low as 46% (men) and 43% (women). An old claim that overweight people carry more water as a percentage has it the wrong way round.
Where the water sits in the body
In a 70 kg man, about 42 litres of the body is water: about 28 litres inside the cells, about 10.5 litres in the spaces around them and about 3.5 litres in the blood (MSD Manual). Organs differ widely. USGS, citing a 1945 study by Mitchell and colleagues, gives the brain and heart as 73% water, the lungs about 83%, skin 64%, muscles and kidneys 79% and bones 31%.
How much water an adult needs
The US National Academies set adequate total water intake at about 3.7 litres a day for men and 2.7 litres for women, about 80% of it from drinks and 20% from food (National Academies, 2004). The European Food Safety Authority considers 2.5 litres a day adequate for men and 2.0 litres for women (EFSA, 2010). Older people tend to feel less thirsty after going without water and drink less, and athletes' performance can drop at as little as 2% dehydration, though mild dehydration does not change thinking in a consistent way (Popkin and others, 2010). We found no primary study behind the often-repeated line that thirst starts at a 2% to 3% loss, so it is not stated here as a figure.
How body water is measured
The reference method is dilution: a person drinks a measured dose of a tracer such as deuterium and the dilution in blood or saliva gives total body water. Prediction equations are built from such measurements, for example Watson's 1980 equations from 458 men and 265 women, which estimate body water from height, weight and age (Watson and others, 1980). Bioimpedance scales estimate it from electrical resistance and are convenient but less exact, which is one reason the published figures for the same age group differ by a few points.
Next: whether your tap water is safe to drink on The Water Hub.
Questions
What percentage of the human body is water?
About 60% in adult men and 50% to 55% in adult women, in the figures USGS and the MSD Manual give, and about 75% to 80% in a newborn.
Why do women have a lower percentage of body water than men?
Mainly because women carry more body fat on average, and fat is only 20% to 30% water against about 72% for lean tissue.
Does the percentage of water in the body fall with age?
Yes. In the 2023 bioimpedance study it fell to 57% in men and 50% in women from age 61, and older adults have measured as low as 46% and 43%.
How much water should an adult drink a day?
The National Academies give adequate total water as about 3.7 litres for men and 2.7 litres for women, from drinks and food together; EFSA gives 2.5 and 2.0 litres.
Sources
- USGS Water Science School: the water in you, read 2026-10-21
- MSD Manual: about body water, read 2026-10-21
- Lu and others, total body water by age and sex, 2023, read 2026-10-21
- Chumlea and others, Fels Longitudinal Study, 1999, read 2026-10-21
- Watson and others, total body water equations, 1980, read 2026-10-21
- National Academies, dietary reference intakes for water, 2004, read 2026-10-21
- EFSA, dietary reference values for water, 2010, read 2026-10-21
- Popkin and others, water, hydration and health, 2010, read 2026-10-21