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Do you really need 8 glasses of water a day?
No study backs a fixed 8 glasses; food and all drinks count, and thirst guides most healthy people well.
What you hear: “Everyone has to drink eight glasses of water daily, and most people walk around chronically dehydrated.”
What the studies found
A 2002 review that went looking for the source of the '8 x 8' rule found no scientific studies supporting it. The US National Academies set adequate intakes of about 2.7 litres a day for women and 3.7 litres for men, but that is total water from all drinks and food, with roughly 20% coming from food, and they said most healthy people meet their needs by drinking to thirst. A 2022 study that tracked water use with isotopes in 5,604 people from 23 countries found needs vary widely with body size, activity, climate and age, and concluded the 8-glass (about 2 litre) advice is not backed by objective evidence. Some surveys using blood and urine tests label many middle-aged and older adults as under-hydrated, but these show links, not proof that drinking more improves health, and they don't support one fixed amount for everyone.
Take care: People who exercise hard, work in heat, are pregnant or breastfeeding, or have certain medical conditions may need more fluid; anyone told by a doctor to limit fluids should follow that advice.
What you could do
Drink when you're thirsty, count tea, coffee and watery foods, and drink more in heat or during hard exercise.
See it in the habit library, with what it means for your own longevity score.
Not medical advice. General information from published studies, not a diagnosis or a treatment plan. Talk with your doctor before starting a supplement or changing a medicine, especially if you are pregnant, take other medicines or have a health condition.
How we judged it
Doesn't hold up: Good studies looked and found no benefit, or the claim is plainly false. We judge the claim as it is usually said online, not the habit itself. Trials assign people at random, so they can show cause; observed studies follow people and show a link, not proof. Last checked: October 2026.
The studies, in their own words
- Valtin, Am J Physiol Regul Integr Comp Physiol 2002
No scientific studies were found in support of 8 x 8.
- National Academies (Institute of Medicine) news release 2004
women who appear to be adequately hydrated consume an average of approximately 2.7 liters (91 ounces) of total water -- from all beverages and foods -- each day, and men average approximately 3.7 liters (125 ounces) daily.
- National Academies (Institute of Medicine) news release 2004
The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide
- Yamada et al. Science 2022
We investigated the determinants of human WT in 5604 people from the ages of 8 days to 96 years from 23 countries
- Yamada et al. Science 2022 (full text)
the common suggestion that we should drink 8 × 8oz glasses of water per day (approx. 2 L) is not backed by objective evidence.
More claims, checked
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- Backed by trialsA 10-minute walk after meals
- Partly trueIntermittent fasting and time-restricted eating
- Partly trueApple cider vinegar before meals
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