You must drink 8 glasses of water a day
No scientific evidence supports the specific recommendation to drink exactly eight 8-ounce glasses of water daily. Hydration needs vary widely by individual, activity level, climate, and diet, and thirst is generally a reliable guide for healthy adults.
What we know
The '8x8' rule (eight 8-ounce glasses per day) has been widely cited but appears to lack a clear scientific origin. A 2002 review by physiologist Heinz Valtin in the American Journal of Physiology searched the literature and found no scientific studies supporting this guideline for healthy adults in a temperate climate. A subsequent 2008 review in the Journal of the American Society of Nephrology similarly found no clear evidence of benefit from drinking increased amounts of water.
The National Academies of Medicine has published Adequate Intake values for total water (including water from all beverages and food), which differ by sex, age, and life stage. These values are considerably higher than the 8x8 rule for some groups and lower for others, and they account for water obtained from food, which can supply roughly 20% of daily water needs.
For healthy individuals, thirst is considered an adequate signal to guide fluid intake. The kidneys are efficient at regulating water balance. However, certain groups may need to pay closer attention, including the elderly (who may have a reduced thirst response), athletes, people in hot climates, and those with certain medical conditions such as kidney stones.
The 'mixed' status reflects that staying well-hydrated is genuinely important for health, and some people under-drink, so the general encouragement to drink water is not harmful. The specific '8 glasses' figure, however, is not evidence-based.
Common claims
- 8 glasses a day is a scientifically established guidelineNo scientific basis found
- Thirst is not a reliable hydration indicatorUnsupported for healthy adults
- Coffee and tea don't count toward fluid intakeLargely false - they do contribute
- More water is always better for healthExcess water can be harmful
Evidence hierarchy
All sources
- Drink at least eight glasses of water a day - really?American Journal of Physiology · 2002
- Fact or Fiction: Must you drink 8 glasses daily?Scientific American · 2009
- Hydration 101: 8 glasses and other myths debunkedUniversity of Rochester Medicine · 2023
- Drink 8 glasses of water a day: fact or fiction?Healthline · 2016