How Much Water Should You Drink Every Day?
The eight-glasses-a-day rule is the most repeated piece of health advice that nobody can source. The Water Intake Calculator gives an estimate based on your body, which is a better starting point than a round number.
Where eight glasses came from
The usual trail leads to a 1945 US recommendation suggesting roughly 2.5 litres of fluid a day - which included the water in food. The following sentence, noting that much of this is met by food, dropped out of the retelling, and what survived was a number with no context.
Current guidance from the European Food Safety Authority is about 2 litres a day of total fluid for women and 2.5 for men, and the US National Academies suggest 2.7 and 3.7. Both of those are total water from all sources, not glasses of plain water.
Food is a substantial share
Roughly twenty per cent of most people's water intake arrives as food. Fruit and vegetables are largely water - cucumber and watermelon are over ninety per cent, an apple around eighty-five. Soup, yoghurt, rice and pasta all carry significant water.
Tea and coffee count too. The idea that caffeine dehydrates you does not survive the research: the diuretic effect at normal intake is smaller than the volume of liquid you just drank, so a cup of tea is a net gain. The same goes for most soft drinks, though they bring other things with them.
What actually changes the number
Body size, because a larger body needs more. Activity and heat, because sweat is the main variable - an hour of hard exercise in warm weather can cost a litre or more. Altitude, which increases losses through breathing. Illness, particularly with fever or stomach upset. And pregnancy and breastfeeding, which raise requirements noticeably.
Thirst is a good guide, and urine colour is better
For most healthy adults, drinking when thirsty works. Thirst is a well-tuned mechanism and it does not need overriding with a schedule.
The simplest check is urine colour: pale straw is about right, dark yellow suggests drinking more, and completely clear suggests you are drinking more than you need. It responds within hours, which makes it far more useful than counting glasses.
It is also possible to drink too much. Hyponatraemia - sodium diluted to dangerous levels - is rare but real, and turns up in endurance events where people drink heavily without replacing salt. Thirst protects against it; a rule that says eight glasses regardless does not.
Common questions
Does coffee count?
Yes. Net positive at normal intake, despite the folklore.
Do I need electrolytes?
Not for ordinary activity. For prolonged heavy sweating - over an hour or two in heat - replacing sodium matters.
Is there a per-kilogram rule?
Around 30 to 35 ml per kilogram of body weight is a common estimate, which is what a calculator is doing. Treat it as a bracket.
What if I am never thirsty?
Thirst sensation declines with age, which is why older adults are advised to drink on a light schedule rather than waiting for it.
Want an estimate for your own size?