The likely source
In a 2002 review in the American Journal of Physiology, Dartmouth’s Heinz Valtin reported finding no scientific study supporting "8×8" (eight 8-ounce glasses, about 1.9 L). The most likely origin he identified was a 1945 Food and Nutrition Board recommendation:
- The advice: about 1 ml of water for each calorie of food, roughly 2.5 L a day.
- The next sentence: "Most of this quantity is contained in prepared foods."
Valtin suggested that if the second sentence was ignored, the advice reads as "drink 2.5 L of water on top of food".
What the 2002 review concluded
- For healthy adults in a temperate climate leading a largely sedentary life, there is no evidence that they must drink eight glasses.
- Caffeinated drinks such as coffee, tea and soft drinks count toward daily fluid intake for most people.
- More fluid is advisable for kidney stone prevention, strenuous exercise, long flights and hot weather.
So eight glasses is not harmful; the problem is giving everyone the same number without evidence.
What to use instead
| Signal | How to use it |
|---|---|
| Thirst | The US National Academies concluded most healthy people meet needs through normal drinking guided by thirst. |
| Your own log | Track a few days and compare with reference intakes. |
| Circumstances | Drink more on hot days, workout days and travel days. |
| Exceptions | Older adults may feel less thirsty, so scheduled drinks work better. |
Counting in glasses anyway
If glasses are easier to think in, settle the size first. A US 8-ounce cup is about 237 ml, so eight is about 1.9 L. To find the size of your own glass, see how many ml in a cup.
Make quick-add buttons match your glass
The quick add buttons on the Today tab (120, 200, 350 and 500 ml by default) can be changed in Settings > Edit quick-add buttons, up to six buttons with your own sizes and drink types. One tap then logs one glass.
Set the goal in glasses
Set the daily target to glass size times number of glasses and the ring tells you how many are left. You can switch the display unit between ml and fl oz in Settings.
FAQ
Does drinking eight glasses improve skin?
The reviews discussed here did not establish a link between water intake and skin. It is better not to expect effects that have not been shown.
Is more than eight glasses even better?
Drinking well beyond your needs adds no benefit, and drinking a lot in a short time can be a problem. See drinking too much water.
Sources: Valtin, Am J Physiol 2002, "Drink at least eight glasses of water a day." Really? · National Academies, 2004 report on water and electrolytes
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