Water Intake Calculator
The “eight glasses a day” rule has no clinical basis and ignores body size entirely. Requirements scale with mass, activity and climate — and roughly a fifth of your intake already arrives in food.
Your Day
Targets are a guide. Thirst and urine colour remain the better everyday signals.
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Where “eight glasses” came from
A 1945 US Food and Nutrition Board note suggested about 2.5 litres of water a day for adults — and added, in the very next sentence, that most of this is contained in prepared foods. The qualifier was dropped and the figure became a rule. It also ignores that a 50 kg adult and a 100 kg adult have very different requirements.
Food supplies about a fifth of it
Fruit, vegetables, soup, yoghurt and cooked grains all carry substantial water. European and US dietary authorities put food at roughly 20–30% of total intake. The figures here separate total fluid from what you actually need to drink, because the second is the number that changes your behaviour.
Coffee and tea count
The idea that caffeinated drinks dehydrate you does not hold at normal intakes. The diuretic effect is mild and outweighed by the fluid volume itself. Moderate coffee and tea contribute positively to hydration. Alcohol genuinely does not.
The better everyday signals
Pale straw-coloured urine and the absence of thirst indicate adequate hydration far more reliably than hitting a target on an app. Overdrinking is a real risk in endurance events: exercise-associated hyponatraemia comes from diluting sodium with excessive plain water, and it is more dangerous than mild dehydration.