How much water should you really drink a day?

August 8, 2026 by AUA Hydrate

“A litre and a half a day.” “Eight glasses.” These figures have been going round so long that nobody questions them any more. Yet they rest on no solid study — and above all they miss the point: what counts is not only how much you drink, it's what your body holds on to.

Where the eight-glasses rule really comes from

It is usually traced back to an American recommendation from 1945, which suggested about 2.5 litres of water a day. The forgotten detail fits in one sentence: the text specified that most of that quantity is contained in prepared foods. That second part disappeared as the advice was passed along, and the figure became a target of glasses to fill.

The result: people forcing themselves to drink, others feeling guilty, and nobody looking at the real indicator.

The serious reference points

The European reference values cover total water intake, food included:

• Adult woman: about 2.0 litres a day.
• Adult man: about 2.5 litres a day.
• A soup, a yoghurt, a cucumber or a watermelon all count. A balanced meal easily supplies 500 to 800 ml.

In practice, that brings the “to drink” share down to around 1.5 to 2 litres for most people — more as soon as it's hot, you're sweating or you go up in altitude.

Why volume isn't enough

You can drink two litres and still be poorly hydrated. Water doesn't stay in the body by magic: it is held there thanks to the dissolved minerals, mainly sodium and potassium. Without them, part of what you drink passes through the body and leaves.

That's the mechanism set out in Why water alone is no longer enough. It explains a situation many people know: drinking a lot, going to the toilet often, and still feeling flat at the end of the day.

The real signals to watch

• The colour of your urine. The simplest and most reliable indicator: pale straw yellow is fine. Dark means drink.
• The end-of-day headache. Often blamed on screens or tiredness, it frequently follows a day with barely any fluid — a link we explore in Cramps and headaches.
• Thirst. It is a late signal: by the time it arrives, the deficit is already there.

The 3 most common mistakes

1. Aiming at a figure rather than at regularity. A litre downed at 6 p.m. does not make up for a dry day: the kidneys flush out the surplus.
2. Counting only plain water. Tea, coffee, soup and fruit all count towards the total. Coffee does not dehydrate at usual doses.
3. Forgetting minerals when it's hot or after exercise. That is precisely the moment when water alone dilutes instead of rehydrating.