Water intake calculator
How much water to drink a day, based on the actual reference intakes.
How it works
Estimates daily water needs from body weight and activity, scaled against the published adequate intakes.
The famous eight glasses a day is folklore. It has no identifiable source in the research literature, and the Institute of Medicine, which sets the US reference intakes, explicitly declines to set a single required volume. Its adequate intake figures are 3.7 litres a day for adult men and 2.7 for women — and crucially, those are total water from all sources.
That last point is what makes most versions of this calculator misleading. Roughly 20% of typical water intake comes from food, and every drink counts, including tea, coffee and juice. The mild diuretic effect of caffeine does not offset the fluid in the drink itself. So the figure to actually drink is meaningfully lower than the total intake figure.
Thirst regulates this well in healthy adults, which is why no authority publishes a hard target. Genuine reasons to pay closer attention are heat, sustained exercise, illness with fever or fluid loss, pregnancy and breastfeeding, and some medical conditions.
This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.
Common questions
- Should I drink eight glasses of water a day?
- There is no evidence base for that figure and no identifiable origin for it. The Institute of Medicine sets adequate intakes of 3.7 L for men and 2.7 L for women, from all sources including food.
- Does tea or coffee count toward water intake?
- Yes. Caffeine’s mild diuretic effect does not cancel out the fluid in the drink, and reference intakes count all beverages.
- How much of my water comes from food?
- Around 20% for a typical diet, which is why the amount to drink is lower than the total intake figure.
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Sources
- Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate — Institute of Medicine, National Academies Press, 2004