Do you need eight glasses of water a day?

"Eight glasses of water a day" is repeated so often it sounds like it must come from somewhere official. When researchers have gone looking for that somewhere, they have struggled to find it.

What the evidence says

Two things are true at once. Staying adequately hydrated matters, that part is not in question. But the specific eight-glasses target is not well supported. Reviews of the topic have traced the rule and come up largely empty-handed: there is no strong body of evidence establishing that healthy adults need exactly that amount, on top of everything else they consume.

The "everything else" is the part the rule ignores. A large share of daily fluid comes from food, fruit, vegetables, soups, and from other drinks, including tea and coffee, which count toward intake rather than against it. Needs also swing widely with body size, activity, heat, and health. A single number cannot fit all of that.

For most healthy people, the body's own signal, thirst, is a reasonable guide, alongside the rough check of pale-coloured urine. That is less tidy than a number, and less marketable, which is probably why the number won.

Where the rule doesn't apply

This is about healthy adults under ordinary conditions. Some medical situations call for deliberately higher or restricted intake, and certain groups, including older adults, whose thirst signal can be blunted, need more attention, not less. Hot climates and hard exercise raise the requirement. Those are exactly the cases to discuss with a clinician rather than a rule of thumb.

The verdict is not "water is bad", an absurd position. It is narrower and more honest: the eight-glasses rule specifically is a tidy number in search of the evidence it is usually credited with.

This is an evidence summary for the general reader, not personalised hydration advice.