The short answer
There is no single amount that suits everyone, but a widely used reference comes from the US National Academies of Sciences, Engineering, and Medicine:
- About 3.7 litres a day of total water for men
- About 2.7 litres a day for women
That is total water from all sources, including food, not just the water you drink. Food usually supplies around a fifth of it. In practice, that means most adults need somewhere around 2 to 3 litres of drinks a day, more in hot weather or when active.
The European Food Safety Authority's reference values are a little lower: about 2.5 litres of total water a day for men and 2.0 litres for women. The two sets of figures are estimates of typical intake in healthy people, not strict targets.
Where "eight glasses a day" came from
The "eight glasses" rule is easy to remember, but it is not based on strong evidence, and it ignores the water in food and other drinks. For some people it is too little, for example in a hot climate or doing manual work. For others it is more than they need. Treat it as a rough reminder, not a rule.
What changes how much you need
| Factor | Effect |
|---|---|
| Hot or humid weather | You lose much more through sweat. In summer heat, needs can rise substantially. |
| Exercise | Drink before and after, and during longer sessions. Heavy sweaters may also need to replace salt. |
| Body size | Larger people generally need more. |
| Pregnancy and breastfeeding | Needs increase. Guidelines add roughly 300 ml a day in pregnancy and around 700 ml a day when breastfeeding. |
| Illness | Fever, vomiting and diarrhoea all increase fluid loss. |
| Older age | The sense of thirst weakens with age, so older adults can become dehydrated without feeling thirsty. |
| Diet | A diet rich in fruit, vegetables, soups and yoghurt supplies more water from food. |
Simple signs you are drinking enough
You do not need to measure every glass. For most healthy adults:
- You rarely feel thirsty.
- Your urine is pale yellow, like straw, rather than dark.
- You pass urine regularly through the day.
Signs you may need more include dark urine, a dry mouth, headaches, tiredness, dizziness and constipation. Severe dehydration causes confusion, a racing heart and very little urine, and needs urgent medical help, especially in children and older people.
Does it have to be plain water?
Water is the best everyday drink because it has no sugar or calories. But other drinks count too:
- Tea and coffee contribute to hydration for regular drinkers, despite the myth.
- Milk hydrates well and adds protein and calcium.
- Fruit juice and soft drinks hydrate but add sugar. Keep them occasional.
- Water-rich foods such as cucumber, watermelon, oranges, yoghurt, lentil soups and curries all contribute.
Alcohol increases fluid loss, so it does not help hydration.
Hydration in hot weather
In a heatwave, or in a hot climate like much of South Asia and the Middle East in summer:
- Drink regularly before you feel thirsty, especially if you work outdoors.
- Replace salt as well as water if you are sweating heavily for hours. Oral rehydration salts (ORS) are cheap and effective, particularly after diarrhoea or vomiting.
- Watch children and older relatives, who are more vulnerable to heat.
- Avoid the hottest part of the day for heavy activity where you can.
Easy habits that help
- Keep a refillable bottle with you, so drinking is effortless.
- Have a glass of water with each meal.
- Drink a glass when you wake up, after a night without fluids.
- Add flavour with lemon, mint or cucumber if plain water bores you.
- Eat more fruit and vegetables, which count towards your total.
Hydration is one part of a healthy routine. Our guides to walking for health and better sleep cover two more of the basics.
This article is general information, not medical advice. If you have a health condition or take medication that affects fluid balance, follow your doctor's guidance.