How to use
- Choose your sex, and whether you are pregnant or breastfeeding.
- Add any moderate activity in the heat today.
- Pick US or European guidance, and litres or US cups.
How it works
There is no exact formula for water needs. This calculator looks up official adequate intakes for healthy adults. These are amounts that should cover almost everyone in a group, not strict targets.
The intakes count total water from drinks and food. Drinks supply about 80%.
| Group | US total | US from drinks | Europe total |
|---|---|---|---|
| Men | 3.7 L | 3.0 L | 2.5 L |
| Women | 2.7 L | 2.2 L | 2.0 L |
| Pregnancy | 3.0 L | 2.3 L | 2.3 L |
| Breastfeeding | 3.8 L | 3.1 L | 2.7 L |
US figures are from the National Academies (2005); European ones from EFSA (2010).
Examples
- Woman, US guidance: about 2.2 L of drinks a day, roughly 9.3 US cups (74 fl oz).
- Same woman, 1 hour of moderate activity in the heat: add 3 to 4 cups, so about 2.9 to 3.1 L.
- Man, US guidance: about 3.0 L of drinks, roughly 12.7 US cups (101 fl oz).
Everyday tips
- Tea, coffee, milk and lower-sugar drinks all count.
- Thirst is a good guide for most healthy people. Aim for pale yellow urine.
- In the heat, the CDC advises 1 cup every 15 to 20 minutes, and no more than 6 cups an hour.
Can you drink too much?
Yes. Your kidneys clear only about 0.7 to 1 litre an hour. Drinking much more can dilute blood sodium, a dangerous condition called hyponatraemia.
Units and the 8-glasses rule
A US cup is 236.6 mL and a US fluid ounce is 29.6 mL. The NHS suggests 6 to 8 cups or glasses of fluid a day, but it doesn’t define a glass size.
We don’t use per-kilogram rules: no official guideline sets one for healthy adults.
If you have heart or kidney disease, or a doctor has limited your fluids, follow their advice. Training for a race? The pace calculator works out your finish time.