Is "8 glasses of water a day" a myth?

2026-10-30

Partly. There is no scientific evidence that every adult must drink eight 8-ounce glasses of plain water a day (the "8x8" rule, about 1.9 liters or 64 fl oz). But the number isn't wildly off either: for many adults it's in the right ballpark for total drinks, as long as tea, coffee, milk and other beverages count too. The myth lies in the details: that it must be plain water, that it's the same for everyone, and that food doesn't count.

Where the rule came from

Nobody knows for sure who first said "eight glasses a day". The most cited lead comes from a 2002 review by Heinz Valtin, a kidney physiologist at Dartmouth Medical School, published in the American Journal of Physiology (abstract on PubMed).

Valtin traced a likely origin to a 1945 recommendation by the US Food and Nutrition Board, which said roughly:

As Dartmouth's press release on the review explains, the last sentence seems to have been forgotten. "2.5 liters, mostly from food" turned into "drink 2.5 liters", and over the decades that became "eight glasses of water".

Other explanations circulate too, such as nutritionists' books in the 1970s and bottled-water marketing, but they're harder to document. What's clear is that the rule was never based on a study showing that people who drink eight glasses are healthier than people who drink less.

What the evidence says

Valtin's review (2002)

Valtin looked for studies supporting 8x8 for healthy adults in a temperate climate doing mostly sedentary work. His conclusion: "I have found no scientific proof that absolutely every person must 'drink at least eight glasses of water a day'." He also pointed out that published research suggested caffeinated drinks should count toward daily intake, and that drinking water you don't need has small downsides of its own (more bathroom trips, cost, and in extreme cases dangerously low blood sodium).

The official reference values

When expert panels later set real intake values, they described total water, from food and all drinks:

The US National Academies (2004) set about 2.7 liters a day for women and 3.7 liters for men, of which roughly 80% comes from beverages, and stated that most healthy people meet their needs by letting thirst guide them (National Academies announcement).
EFSA (2010) set 2.0 liters for women and 2.5 liters for men, also total water.

Neither panel said "eight glasses of plain water". For the full picture of these numbers, see how much water you should drink a day.

Measuring what people actually use (2022)

A study published in Science measured water turnover in more than 5,600 people aged 8 days to 96 years using labeled water. Daily turnover ranged widely, from about 1 to 6 liters, depending mostly on physical activity, body size, sex, age and climate. One of the authors put it bluntly: the science "has never supported the old eight glasses thing as an appropriate guideline" (University of Wisconsin–Madison summary).

So is eight glasses right or wrong?

It depends on what you mean by "glasses" and who you are.

Roughly right, as a rough guide. If you count all drinks, 64 fl oz (1.9 liters) is close to what European guidelines imply for women from beverages, and a bit low for many men. The UK's NHS still uses a similar rule of thumb: 6 to 8 cups or glasses of fluid a day, and tea and coffee count.

Too little for some people:

people who exercise a lot or work physically
anyone in hot or humid weather
larger adults
breastfeeding mothers
anyone with fever, vomiting or diarrhea

Possibly more than needed for others:

small, sedentary adults in a cool climate
people who eat a lot of fruit, vegetables and soup

Not for everyone at all: people with heart failure, advanced kidney disease or some other conditions may be told to restrict fluids. Their doctor's number wins over any rule of thumb.

What the myth gets wrong

The real problem with 8x8 isn't the number. It's the hidden assumptions:

"It has to be plain water." It doesn't. Water is the best everyday choice because it has no sugar or calories, but tea, coffee and milk also hydrate. See do coffee, tea, milk and juice count?.
"Food doesn't count." It does, for roughly a fifth of the total.
"Everyone needs the same amount." Needs vary several-fold between people and between days.
"If you're thirsty, you're already dehydrated." For healthy adults, thirst kicks in early and is a useful signal, not a sign of damage. It does become less reliable with age.
"More is always better." Drinking far more than you need doesn't add benefits, and in extreme cases, like drinking liters in a short time during endurance events, it can be dangerous. See can you drink too much water?.

There are more popular hydration myths. We collect them in 10 hydration myths, debunked.

A better rule than "eight glasses"

Instead of a fixed number, use a personal target and check it against your body:

Start with a goal that fits you, for example one based on your body weight, and raise it on active or hot days.
Count everything you drink, not just plain water.
Let thirst and urine color guide you: pale yellow most of the day is a good sign.
Spread drinks over the day rather than catching up at night.

If eight glasses is an easy mental anchor for you, that's fine. Glasses are a handy unit. Water Tracker can show your intake in glasses, milliliters or ounces, whichever you think in, and sets your goal from your weight and activity rather than a one-size-fits-all number.

Bottom line

The 8x8 rule probably comes from a 1945 recommendation whose key line, "most of this quantity is contained in prepared foods", got lost.
No study shows that every adult needs eight glasses of plain water.
Counting all drinks, 6–8 glasses is a reasonable starting point for many adults, but your real needs depend on size, activity, climate and health.
Drink to thirst, aim for pale urine, and adjust. If you have a medical condition, follow your doctor's advice.

This article is general information, not medical advice. If you have a health condition, are pregnant or take medication, ask your doctor how much you should drink.