Myth Check: Do You Really Need Eight Glasses of Water a Day?

“Drink eight glasses of water a day” is advice almost everyone has heard and almost no one questions. It’s printed on water bottles, repeated by fitness apps and passed around as if it were settled science.

It isn’t, though it isn’t entirely wrong either. Here’s what the evidence says.

Our rating: oversimplified. Water matters, but there’s no single number that suits everyone, and a lot of your water doesn’t come from a glass.

Where did “8 × 8” come from?

Nobody knows for certain. In 2002, Dr Heinz Valtin, a kidney specialist at Dartmouth Medical School, went looking for the scientific basis of the eight-glasses rule. He couldn’t find one.

The closest candidate he identified was a 1945 recommendation from the US Food and Nutrition Board, which suggested about 1 millilitre of water for each calorie of food, or roughly 2.5 litres a day for many adults. But the very next sentence noted that most of this quantity is already contained in prepared foods. Over the decades, that second sentence seems to have been forgotten.

Valtin concluded that there was no scientific evidence that healthy adults in a temperate climate, who are not very active, need to drink eight glasses of water a day on top of what they get from other drinks and food.

How much water do we actually need?

In 2004, the US National Academies reviewed the evidence and set Adequate Intakes for total water (from all drinks and food) of about:

• 3.7 litres a day for men
• 2.7 litres a day for women

That sounds like a lot until you notice two things. First, around 20% of our water typically comes from food: fruit, vegetables, soups, yogurt and cooked grains are mostly water. Second, the panel concluded that most healthy people meet their daily needs by letting thirst guide them, along with drinking at meals.

A large study published in Science in 2022 added real-world detail. Researchers measured water turnover (how much water the body uses and replaces each day) in more than 5,600 people from 26 countries, from newborns to 96-year-olds. The amounts varied enormously with body size, physical activity, age, climate and altitude. The authors concluded that a one-size-fits-all “eight glasses” guideline isn’t supported: many people need less from drinks, and some (very active people, or those in hot climates) need more.

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Does coffee count?

Yes. A common add-on to the eight-glasses myth is that coffee and tea “don’t count” because caffeine makes you urinate. In a 2016 randomised trial, researchers compared how well 13 different drinks kept people hydrated over four hours. Coffee, tea and cola performed about the same as still water. Milk and oral rehydration solution actually retained more fluid than water.

That doesn’t make every drink a good choice (sugary drinks bring their own problems), but your morning coffee does contribute to your fluid intake.

Who should pay closer attention?

Thirst works well for most healthy adults, but not for everyone. It’s worth being more deliberate about fluids if you are:

• Older. The sense of thirst tends to weaken with age, so it can lag behind the body’s needs.
• Exercising hard, or in hot weather. Sweat losses can be large.
• Ill with fever, vomiting or diarrhoea.
• Pregnant or breastfeeding, when requirements rise.
• Prone to kidney stones, where clinicians often advise drinking more.

On the other hand, some people, for example those with certain heart or kidney conditions, are told to limit fluids. If that’s you, follow your clinician’s advice rather than any general rule.

A simpler rule of thumb

Instead of counting glasses, try this:

1. Drink when you’re thirsty, and with meals.
2. Check the colour. Pale yellow urine generally suggests you’re well hydrated; dark urine suggests you need more.
3. Make water your default drink. As Harvard’s Nutrition Source points out, water hydrates without adding calories or sugar. The best reason to reach for it isn’t a magic number, but what it replaces.
4. Drink more when you sweat more.

The verdict

Eight glasses a day isn’t harmful for most people, and if it helps you choose water over sugary drinks, it’s a useful habit. But it isn’t a scientific requirement. Your needs depend on your body, your day and your plate. For most healthy adults, thirst is a surprisingly good guide.