Articles · Nutrients

How much water do you really need?

Where "eight glasses a day" came from, how much water adults actually need, why food and tea count, and the simple signs that you're drinking enough.

2026-09-30 · 4 min read

"Drink eight glasses of water a day." You've heard it, probably more than once. But who said so, and is it true? Do you need to carry a bottle everywhere, or is thirst a good enough guide?

Where did "eight glasses" come from?

Nobody is quite sure. A physiologist who went looking for the evidence behind "at least eight glasses a day" in 2002 found no scientific studies supporting it. One likely origin is an American recommendation from 1945 that adults need about 2.5 litres of water a day, a sentence that went on to say most of it is already contained in food. That second part was quietly forgotten.

Think about it: How much did you drink yesterday, in glasses? And how much of your water do you think came from food?

How much do adults need?

Expert bodies give these totals for water from all drinks and food:

In the United States: about 3.7 litres a day for men and 2.7 litres for women.
In Europe: about 2.5 litres a day for men and 2.0 litres for women.

Food usually provides around a fifth of that, more if you eat lots of fruit, vegetables and soups. So from drinks, most adults need somewhere around 1.5–3 litres: roughly 6–12 glasses of 250 ml, depending on size, climate and activity.

What counts?

Water, of course: the best everyday choice, with no sugar or calories.
Tea and coffee count too. Caffeine has only a mild effect on urine, and regular drinkers get used to it.
Milk, and soups, and watery foods such as cucumber, melon, oranges and tomatoes.
Juice and sugary drinks count as water, but they bring a lot of sugar with it; keep juice to a small glass a day.

Am I drinking enough?

For most healthy adults, thirst is a reliable guide. Two simple checks:

Your urine should be pale straw-coloured. Dark yellow, strong-smelling urine suggests you need more.
How you feel: headaches, tiredness and trouble concentrating can be early signs of being short of fluid.

You'll need more in hot weather, when you exercise, when you have a fever, vomiting or diarrhoea, and when you're pregnant or breastfeeding. Older people often feel less thirsty, so it's worth drinking regularly even without a strong urge.

Try this: Keep a glass or bottle where you can see it: on your desk, by the cooker, next to the bed. If you track water on SustainBite, My Plan counts it in 250 ml glasses.

Can you drink too much?

Yes, though it's rare. Drinking very large amounts in a short time, as some endurance athletes have, can dilute the sodium in the blood to a dangerous level. For everyday life, drinking to thirst is enough.

Think about it: Do you reach for a snack when you might actually be thirsty? Try a glass of water first and see.

Quick quiz

Is there good scientific evidence for "eight glasses a day"?

No. A 2002 review found no studies supporting it. How much you need depends on your size, your activity, the climate and how much water your food contains.

Do tea and coffee count towards your fluid intake?

Yes. Their mild effect on urine doesn't cancel out the water they provide, especially for regular drinkers.

What's the simplest everyday sign you're drinking enough?

Pale straw-coloured urine, and not feeling thirsty.

Sources

Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? American Journal of Physiology, 2002.
Popkin BM, D'Anci KE, Rosenberg IH. Water, hydration, and health. Nutrition Reviews, 2010.
National Academies. Dietary Reference Intakes for water, potassium, sodium, chloride, and sulfate, 2005.
European Food Safety Authority. Scientific opinion on dietary reference values for water, 2010.

General information, not medical advice. If you have a health condition, are pregnant or take medication, ask a doctor or dietitian what is right for you.