TempoLife › Features › Nutrition myths, checked › eight-glasses-of-water
You must drink eight glasses of water a day.
The target is real in spirit and wrong in the specifics: it is the wrong volume, it ignores food and every other drink, and it is the same number for a 50 kg office worker and a 100 kg roofer in August.
Verdict: It depends8 glasses of waterhydrationwater intakehow much water
The claim, as people state it
“You must drink eight glasses of water a day.”
What is actually known
European food-safety guidance puts adequate total water intake at roughly 2.0 litres a day for adult women and 2.5 litres for adult men. Crucially, that is total water: what is in food, what is in tea, coffee, milk, soup and juice, and what comes out of the tap. Food alone typically supplies a fifth to a third of it.
The eight-glasses rule counts only plain water, and eight 240 ml glasses is less than the total-water figure it is usually quoted as satisfying. So the rule simultaneously asks for too much of one category and ignores most of the actual supply. Tea and coffee count towards hydration; the diuretic effect of ordinary caffeine doses does not cancel the fluid they arrive in.
Requirements vary enormously with body size, activity, air temperature, humidity, altitude, illness and some medications. A fixed number cannot cover that range, which is why guidance bodies publish population averages and then say to drink to thirst.
Why the myth persists
Because it is a number, and health advice without a number feels like no advice at all. The specific figure appears to descend from a decades-old recommendation that did mention food as a source, with the qualifying sentence dropped in retelling — a rule that survived by losing its own footnote.
The arithmetic
Eight glasses at 240 ml is 1.92 litres. European food-safety guidance puts adequate total water intake at about 2.0 litres a day for women and 2.5 for men — and that figure counts water in food and in every drink, not just what comes out of a tap. So the rule asks for less than the target it is quoted as meeting, while ignoring most of the supply.
| Drunk over a day | Fluid it supplies | kcal it costs |
|---|---|---|
| 5 mugs of green tea | 1.25 L | 13 |
| 3 mugs of black coffee | 0.75 L | 15 |
| Both, in one day | 2.00 L | 28 |
Five mugs of tea and three of coffee is 2.00 litres — most of an adult's daily requirement — for 28 kcal in total. Under the eight-glasses rule, none of it counts. That is the specific way the rule is wrong: not the spirit, the accounting.
Then there is body size, which the rule ignores entirely. Requirements scale with mass, activity and air temperature, and a single fixed number cannot cover a 50 kg office worker and a 100 kg roofer in August. Thirst does cover both, and is free.
Every figure in this section was computed when this page loaded, from 5 237 food rows and 31 exercise rows already in this repository. Nothing in it is stored as prose.
The honest nuance
Thirst is a good guide for most healthy adults, and urine colour is a reasonable free check — pale straw rather than clear or dark. Older adults are a real exception: thirst sensation declines with age, and deliberate scheduling genuinely helps.
Drinking far more than needed is not automatically safe. Sustained very high intake without electrolytes can dilute blood sodium, which is a medical emergency. It is rare and it is not a reason for caution at ordinary intakes, but "more is always better" is not true here either.
This claim is rated "it depends" because the evidence is genuinely mixed, not to avoid taking a position. Where good researchers still disagree, this page says so rather than choosing a side for you.
General information, not a diagnosis and not medical advice. Anything touching a condition you have, a medicine you take or a test result belongs with your own clinician, who has information no web page does.
Frequently asked questions
Does coffee dehydrate you?
Not at ordinary intakes. The mild diuretic effect of caffeine does not exceed the fluid the drink delivers, so a mug of coffee is a net positive for hydration.
Should I drink more when exercising?
Yes — sweat losses are real and can exceed a litre an hour in heat. Drink to thirst during, and replace deliberately afterwards if the session was long or hot.
Source: USDA FoodData Central (SR Legacy) via the TempoLife food database; MET values from the 2011 Compendium of Physical Activities · checked 2026-09-02
Work it out for yourself
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Stop guessing at your own numbers
Almost every myth on this site ends at "it depends on your total intake and your total movement". TempoLife records both, so the answer stops being a debate.