Water Intake Calculator

How much water do you actually need?

Where "8 glasses a day" came from

The rule appears to trace back to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres of water a day. The very next sentence noted that most of this quantity is contained in prepared foods — and that sentence was steadily dropped as the advice was repeated.

A 2002 review in the American Journal of Physiology searched for evidence supporting the eight-glasses rule and found none. It has persisted because it is memorable, not because it is right.

This does not mean hydration is unimportant. It means the correct amount depends on your size, your activity, the temperature and what you eat — which is what the calculator above tries to reflect.

Better indicators than a number

Note that B vitamins and some medications turn urine bright yellow regardless of hydration, so use the colour test alongside how you feel rather than on its own.

It does not have to be water

All drinks contribute to hydration, including tea and coffee. The idea that caffeine dehydrates you is overstated: at habitual intakes the diuretic effect is mild and more than offset by the fluid in the drink itself.

Food contributes substantially too. Cucumber, lettuce and watermelon are over 90% water; soup, yoghurt and most fruit are well over half.

Two genuine cautions. Alcohol does have a meaningful diuretic effect, so alcoholic drinks are a net negative for hydration. And drinking far too much water too quickly can cause hyponatraemia — dangerously diluted blood sodium — which is rare but serious, and mostly affects endurance athletes.