Drinking two litres of water a day to stay healthy and hydrated is a myth, with people needing up to six litres depending on job, climate and sex, scientists have found.
In recent decades, the need to drink eight glasses of water a day has become standard advice, yet there is little evidence to back it up.
Researchers at the University of Wisconsin-Madison in the US studied thousands of people from 26 countries to find out how much water they needed, and discovered it varied widely.
They found that daily averages ranged from as little as one litre per day to six litres, which included water from other drinks like tea and coffee, and also water in food.
"The science has never supported the old eight glasses thing as an appropriate guideline, if only because it confused total water turnover with water from beverages and a lot of your water comes from the food you eat," said Prof Dale Schoeller, emeritus professor of nutritional sciences.
"But this work is the best we've done so far to measure how much water people actually consume on a daily basis - the turnover of water into and out of the body - and the major factors that drive water turnover."
Unlike previous studies which had asked people to self-report water intake, researchers measured water as it moved through the body.
Participants drank special "trackable" water containing hydrogen and oxygen isotopes, so that scientists could tell when it had passed through.
They found big differences depending on temperature, sex and levels of physical activity.
Weight gain and exercise a major factor
For example, a 20-year-old man, weighing 11 stone who lived at sea level in a developed country like Britain where the mean air temperature was 10C (50F), and who did average physical activity, would need around 3.2 litres per day.
A nine stone woman of the same age and activity level, living in the same area, would need just 2.7 litres.
When people doubled their energy expenditure in a day they required an extra one litre, the researchers found, while a 50 per cent increase in humidity required an extra 0.3 litres a day.
Weight gain was also a major factor, with the average eight stone person using around 2.5 litres a day while the average 15 stone person used five litres.
Athletes were found to use about a litre more than non-athletes. One male athlete in the study was found to use 10 litres of water a day, although experts admitted he was an outlier.
The researchers found hunter-gatherers, mixed farmers and subsistence agriculturalists all had higher water needs than people who live in industrialised economies.
"Those people in low human development index countries are more likely to live in areas with higher average temperatures, more likely to be performing physical labour, and less likely to be inside in a climate-controlled building during the day," added Prof Schoeller.
"That, plus being less likely to have access to a sip of clean water whenever they need it, makes their water turnover higher."
Origins of eight glasses a day
The eight glasses a day rule appears to originate from Dr Fredrick J Stare, the American nutritionist who, in 1974, suggested a figure of six to eight glasses. He said this could include coffee, tea, milk, soft drinks and beer alongside the water content of fruit and vegetables.
Most nutritionists now accept that the body controls water levels well, with the body urinating out what it does not need and triggering thirst when it requires a top up.
Experts argue that encouraging people to drink more water than the body asks for is the equivalent of consciously breathing more, simply because oxygen sustains life.
Drinking too much water can be dangerous. If the kidneys cannot get rid of the excess, it dilutes the sodium content of the blood, triggering a condition called hyponatremia, which can be life-threatening.
The University of Wisconsin-Madison's research was published in the journal Science.