Water intake calculator
A daily fluid target based on your weight, how active you are and your climate. The widely repeated eight-glasses rule has no real evidence behind it — needs scale with body size and conditions, which is what this accounts for.
Built by Mubashir. Uses a published standard formula, named below. Not medical advice — see the limitations section.
About you
Daily fluid target
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Glasses of 250 ml
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US cups
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Base requirement
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Added for conditions
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Show the working
—Talk to a doctor before following this if you have a heart, kidney or liver condition. Several conditions and some medications require fluid restriction, and in those cases drinking to a general target can be actively harmful. This calculator applies a population rule of thumb and knows nothing about your medical history.
Where the eight-glasses rule came from
It is not from a study. The figure is usually traced to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres of water a day — which included, in a sentence that got widely dropped, the note that most of this is contained in prepared foods.
The surviving half of that advice became a rule that everyone should drink eight glasses on top of what they eat and drink otherwise. It has persisted for eighty years without ever having had much behind it.
What this calculator uses instead
Roughly 35 millilitres per kilogram of body weight, which is a common clinical rule of thumb for adults and at least scales with body size. Adjustments are then added for training, hot climates, pregnancy and breastfeeding.
This is still a rule of thumb. It is a better one than a fixed eight glasses, because a 55 kg person and a 100 kg person plainly do not need the same amount, but it remains a starting estimate rather than a measured requirement.
Food and other drinks count
Around a fifth of typical fluid intake comes from food. Fruit, vegetables, soup, yoghurt and rice all carry substantial water. Watermelon and cucumber are over ninety percent water by weight.
Tea and coffee count too. The belief that caffeine dehydrates you does not survive contact with the evidence — the diuretic effect at normal consumption levels is smaller than the volume of fluid the drink provides, so a cup of coffee is a net positive for hydration. The same is true of most soft drinks, though they bring sugar with them.
Alcohol is the genuine exception, since it suppresses the hormone that limits urine production.
Thirst is a better guide than arithmetic
For healthy adults with normal kidney function, thirst is a well-calibrated signal and drinking to it works. The main exceptions are older adults, whose thirst response blunts with age, and endurance athletes, where sweat losses can outpace thirst during prolonged effort.
Urine colour is the most practical check available. Pale straw suggests adequate hydration; consistently dark yellow suggests drinking more. Bear in mind that some vitamin supplements, particularly B vitamins, turn urine bright yellow regardless of hydration.
Drinking too much is possible
Rare, but real. Consuming large volumes rapidly can dilute blood sodium to dangerous levels, a condition called hyponatraemia. It has occurred in endurance events and in water-drinking contests, and it can be serious.
The practical guidance is to spread intake through the day rather than consuming large amounts at once, and to include electrolytes during prolonged intense exercise rather than water alone.
Frequently asked questions
Is eight glasses a day accurate?
There is no strong evidence for it. The figure appears to derive from a 1945 recommendation that also noted most of that fluid comes from food — a qualification that got lost. Needs scale with body size, activity and climate.
Does coffee count toward my intake?
Yes. The diuretic effect of caffeine at ordinary consumption levels is smaller than the fluid volume the drink provides, so coffee and tea are net contributors to hydration.
How do I know if I am dehydrated?
Urine colour is the most practical indicator — pale straw suggests adequate hydration, consistently dark suggests more is needed. Thirst, fatigue and headache are other signals. B vitamin supplements colour urine regardless.
Can I drink too much water?
Yes, though it is uncommon. Consuming large volumes rapidly can dilute blood sodium dangerously, a condition called hyponatraemia. Spread intake through the day rather than drinking large amounts at once.
Does food count toward fluid intake?
Roughly twenty percent of typical intake comes from food. Fruit, vegetables, soup and yoghurt all carry significant water.
Should I drink more when exercising?
Yes, and the calculator adds for it. During prolonged intense exercise, particularly in heat, include electrolytes rather than water alone, since sodium is lost in sweat.