How much water do you need?
Water makes up much of the body and is lost all day through breath, sweat and the bathroom, so it has to be replaced. The well known eight glasses a day rule is easy to remember, but it is not the official figure, and it leaves out the water that comes from food.
This water intake calculator shows the official adult reference for total water a day, from all drinks and food, in litres, US fluid ounces and 8 ounce cups. Add what you have had so far and it shows what is left.
How to use it
- Choose women, men, or women who are breastfeeding.
- Read the daily reference in litres, ounces and cups.
- Optionally type how much you have drunk so far today, in cups, ounces, litres or millilitres.
- Read how much is still to go. Remember that food, especially fruit, vegetables and soup, supplies some of it.
The reference amounts
men: 3.7 litres of total water a day
women: 2.7 litres a day; breastfeeding: 3.8 litres a day
1 US fluid ounce = 29.5735 mL; 1 cup = 8 fluid ouncesThese are the Adequate Intakes set by the Institute of Medicine in 2004 for healthy adults, as reported by the CDC. They cover water from every source: plain water, other drinks and the water in food. They are based on what healthy people typically take in, so they are a reference, not a strict target.
Worked example: a man who has had 1.5 litres
- Reference for men: 3.7 litres a day
- In ounces: 3.7 ÷ 0.0295735 = 125 oz, about 15.6 cups
- Had so far: 1.5 L
- Still to go: 3.7 − 1.5 = 2.2 L, about 74 oz
The daily reference for men is 3.7 litres, or 125 oz, and with 1.5 litres so far, 2.2 L remains. Part of that will come from meals, so the amount left to drink is usually less.
The references in other units
| Group | Litres | US fluid ounces | 8 oz cups |
|---|---|---|---|
| Women | 2.7 | 91 | 11.4 |
| Men | 3.7 | 125 | 15.6 |
| Breastfeeding | 3.8 | 128 | 16.1 |
Hot weather, exercise, illness with fever, and high altitude all raise how much water the body loses. Older adults may feel less thirsty even when they need fluid.
Why the reference is not a strict target
The Adequate Intakes are based on how much healthy people typically take in, not on a precise requirement, because the body is good at balancing water through thirst and the kidneys. Healthy adults who drink when thirsty and with meals usually meet their needs.
Simple ways to keep up
- Keep a bottle where you can see it and refill it a set number of times.
- Pale yellow urine is a common sign of drinking enough.
- Water, milk and unsweetened drinks are better everyday choices than sugary drinks.
- For your daily energy needs, see the calorie calculator.
Frequently asked questions
How much water should I drink a day?
The official adult reference is about 3.7 litres of total water a day for men and 2.7 litres for women, counting all drinks and the water in food. Food usually provides part of it, so the amount you drink is less. Needs go up with heat, exercise and breastfeeding.
Is eight glasses of water a day right?
Eight 8 ounce glasses is 64 ounces, about 1.9 litres. It is an easy rule of thumb but not an official figure. The official references for total water are higher, at 2.7 litres for women and 3.7 for men, though part of that comes from food rather than drinks.
Do coffee, tea and food count toward water intake?
Yes. The reference amounts count water from all drinks, including coffee, tea and milk, and from food, such as fruit, vegetables and soup. Plain water is still a good everyday choice, since it has no sugar or calories. Sugary drinks add water but also many calories.
Can you drink too much water?
It is rare, but drinking a very large amount in a short time can lower the salt level in the blood, which can be dangerous. It is most often seen in long endurance events. Drinking steadily through the day, guided by thirst, is the safe approach for most healthy adults.
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