Water intake calculator
Get a daily water target based on your weight, how active you are and how hot your climate is. The popular eight-glasses rule has little evidence behind it, because the amount you need depends on your body size and conditions. This calculator takes both into account.
Uses a standard published formula, explained below. The result is an estimate, not medical advice, and its limits are explained further down the page.
About you
Daily fluid target
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Glasses of 250 ml
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Same in fluid ounces
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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 did not come from a study. The idea is usually traced to a 1945 recommendation from the US Food and Nutrition Board of about 2.5 liters of water a day. The same advice added that most of this water is already contained in food, but that part was widely forgotten.
What remained became the rule that everyone should drink eight glasses of water on top of everything else they eat and drink. It has lasted for about eighty years with very little evidence to support it.
How this calculator works out your target
It starts with about 35 milliliters of water for each kilogram of body weight, which is a common guideline for adults and at least grows with body size. It then adds 500 ml for a hot or humid climate, 300 ml for pregnancy or 700 ml for breastfeeding, plus an amount for exercise based on your activity level.
This is still a rule of thumb. It is better than a fixed eight glasses, because a person weighing 55 kg and a person weighing 100 kg clearly do not need the same amount, but it is a starting estimate rather than a measurement.
Food and other drinks count too
About a fifth of the water you take in each day comes from food. Fruit, vegetables, soup, yogurt and rice all contain a lot of water, and watermelon and cucumber are more than 90% water by weight.
Tea and coffee also count. The belief that caffeine dehydrates you is not supported by the evidence. At normal amounts, the extra urine caffeine causes is less than the fluid the drink provides, so a cup of tea or coffee still adds to your daily intake. The same is true of most soft drinks, although they add sugar as well.
Alcohol is the real exception, because it blocks the hormone that helps your body hold on to water.
Thirst is usually a good guide
For healthy adults with normal kidneys, thirst is a reliable signal, and drinking when you are thirsty works well. The main exceptions are older adults, who often feel less thirsty as they age, and endurance athletes, who can lose sweat faster than thirst can keep up during long events.
The color of your urine is the easiest check. Pale straw yellow suggests you are drinking enough, while dark yellow most of the time suggests you should drink more. Some vitamin supplements, especially B vitamins, turn urine bright yellow whatever your hydration.
Drinking too much water is possible
It is rare, but it happens. Drinking a very large amount of water in a short time can dilute the sodium in your blood to a dangerous level. This condition is called hyponatremia, and it has occurred during endurance races and water-drinking contests.
The simple way to avoid it is to spread your drinking through the day instead of drinking a large amount at once. During long, intense exercise, use a drink with electrolytes rather than plain water alone.
Frequently asked questions
How much water should I drink a day?
A common starting point for adults is about 35 ml for each kilogram of body weight, so around 2.5 liters for someone who weighs 70 kg. You need more in hot weather, when you exercise and during pregnancy or breastfeeding.
Is eight glasses of water a day right for everyone?
No. There is little evidence for that figure, which seems to come from a 1945 recommendation that also said most of that water comes from food. How much you need depends on your size, your activity and your climate.
Does tea or coffee count toward my water intake?
Yes. At normal amounts, the extra urine caused by caffeine is less than the fluid in the drink, so tea and coffee still add to your daily intake.
How do I know if I am dehydrated?
The color of your urine is the easiest sign: pale straw yellow suggests you are drinking enough, while consistently dark yellow suggests you need more. Thirst, tiredness and headaches are other signs.
Can I drink too much water?
Yes, although it is rare. Drinking a very large amount quickly can lower the sodium in your blood to a dangerous level, a condition called hyponatremia. Spread your drinking through the day.
Does food count toward my fluid intake?
Yes. About a fifth of the water most people take in each day comes from food, especially fruit, vegetables, soup and yogurt.
Should I drink more when I exercise?
Yes, and the calculator adds extra for exercise. During long or intense exercise, especially in the heat, use a drink with electrolytes as well as water, because you lose salt in your sweat.