Nobody proved you need eight glasses of water

6 November 2026, 2 minute read

The rule is everywhere, and nobody can say where it came from.

The eight glasses rule is repeated on bottles, in apps and in doctors' waiting rooms. It has no clear scientific origin. The bodies that actually set intake values give ranges, and they count far more than plain water.

The US National Academies put adequate total water at about 3.7 litres a day for men and 2.7 litres for women. The European Food Safety Authority says 2.5 and 2.0 litres. Both figures are for total water, which means every drink plus the water inside food. Food supplies roughly a fifth of it. Soup, fruit, vegetables, yogurt and cooked grains are mostly water.

What counts

Tea and coffee count. Caffeine makes you pass a little more water, but not as much as the cup contains, so the net effect is still hydrating. Milk counts. Sparkling water counts. Sugary drinks count too, although you pay for it in sugar.

How to tell if you drink enough

For a healthy adult, thirst is a good guide. Urine the colour of pale straw means you are fine; dark yellow means drink something. You need more in hot weather, during long exercise, and when you have a fever or a stomach bug.

Thirst becomes less reliable with age. If you are looking after someone over seventy, offering a drink at set times works better than waiting for them to ask.