Do You Really Need 8 Glasses of Water a Day?
Not necessarily. A 2002 review found no scientific studies supporting the "8 × 8" rule of eight 8-ounce glasses a day, and the US National Academies say they offer no rule of thumb based on glasses at all. Eight glasses, about 1.9 liters (64 fl oz), is still close to the UK's NHS guide of 6 to 8 cups or glasses of fluid a day, so it can work as a rough reminder, as long as you count other drinks and drink more when you need to.
Where the 8 glasses rule comes from
Nobody can say for certain where "8 × 8" began. Part of its staying power is that it is easy to remember: eight glasses of 8 fl oz each, which adds up to 64 fl oz, or about 1.9 liters.
One piece of early official advice is worth knowing. According to a 2012 paper in the Journal of Medical Case Reports, the US Food and Nutrition Board recommended a daily water intake of 2.5 liters (about 85 fl oz) in 1945, and concluded that most of this amount is contained in prepared foods.
Read without its second half, that sounds like a drinking target. Read in full, it describes total water, much of which comes from food. Whether this is how the eight-glasses rule started is not known, but it shows how easily a total-water figure can be mistaken for a drinking target.
What the evidence says
In 2002, Heinz Valtin looked for the science behind the rule and published his review in the American Journal of Physiology. His conclusion was blunt: "No scientific studies were found in support of 8 x 8."
He was careful about what that means. His conclusion covered only "healthy adults in a temperate climate leading a largely sedentary existence". He also noted that caffeinated drinks "may indeed be counted toward the daily total", and that larger intakes are advisable in some diseases and for people doing hard work or exercise in hot weather.
So the finding is not that eight glasses is harmful or wrong. It is that nobody has shown that every healthy adult needs that exact amount.
What health authorities recommend instead
Rather than counting glasses, the major health authorities give adequate intakes of total water, which means drinks and food together. Here is how they compare with the rule:
| Source | Daily figure | What it covers |
|---|---|---|
| "8 × 8" rule | 8 glasses, about 1.9 L (64 fl oz) | a rule of thumb, not backed by studies |
| NHS (UK) | 6 to 8 cups or glasses | all fluids, as a guide; cup size not defined |
| EFSA (Europe) | women 2.0 L (68 fl oz), men 2.5 L (85 fl oz) | total water, including food |
| NASEM (US) | women 2.7 L (91 fl oz), men 3.7 L (125 fl oz) | total water, including food |
| NASEM (US), drinks part | women about 2.2 L (74 fl oz), men about 3.0 L (101 fl oz) | drinks of all kinds |
Two things stand out. First, the US figures for drinks alone, roughly 9 cups for women and 13 for men in NASEM's own words, are higher than eight glasses. Second, they are still not personal targets. They are adequate intakes for healthy people, and NASEM adds that "the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide." There is more on what these numbers mean in how much water should you drink a day.
The US National Academies are explicit about glasses: "We don't offer any rule of thumb based on how many glasses of water people should drink each day."
Do coffee, tea and food count?
Yes. The NHS says that water, lower-fat milk and lower-sugar drinks, "including tea and coffee, all count as part of your daily intake." The US National Academies found that about 80% of the water people take in comes from drinks, including caffeinated ones, and that food supplies about 19 to 20%. The details are in does coffee count as water intake? and in our table of foods with high water content.
So if you count only glasses of plain water, you are leaving out a good part of what you actually take in.
Why eight glasses can still be a handy guide
Rules of thumb last because they are easy, and this one has a few things going for it:
- It is simple to count, whether you use a glass, a mug or a bottle.
- It lands close to the NHS guide of 6 to 8 cups or glasses of fluid a day.
- It nudges you to spread drinks out, which matches the NHS tip to "drink regularly throughout the day".
It works best as a loose reminder, not a target to hit at any cost, because needs vary:
- You may need more if you are pregnant or breastfeeding, in a hot environment, active for long periods, or ill or recovering from illness, according to the NHS. NASEM notes that very active people who are continually exposed to hot weather often need 6 liters (about 203 fl oz) or more of total water a day.
- You may need less from glasses on a cool, quiet day when you also drink tea, coffee or milk and eat plenty of watery foods.
- Your doctor's number comes first. If a doctor has told you to limit fluids (for example for heart failure or kidney disease), follow their limit rather than general advice.
A better way to think about it
Instead of a fixed count of glasses, try this:
- Start from the adequate intake for your group, and remember it includes food.
- Count every drink, not just plain water.
- In everyday conditions, let thirst guide you. It is a weaker guide in older age, since NASEM notes that "thirst and fluid intake are impaired in the elderly", and babies and young children rely on adults to offer drinks.
- Check the color of your pee. The NHS says most people should aim for "a clear pale yellow colour".
If you like having a number to aim for, Hydrolotl suggests a daily goal based on EFSA's guidance that you can change, and lets you log tea, coffee and milk as well as water.
FAQ
How many ounces is 8 glasses of water?
Eight glasses of 8 fl oz each come to 64 fl oz, which is about 1.9 liters. If your glasses hold 250 ml (about 8.5 fl oz), eight of them make 2 liters (about 68 fl oz).
Is drinking 8 glasses of water a day too much?
For most healthy adults, no. NASEM set no upper limit for water, because healthy people in temperate climates normally regulate how much they drink. Acute water intoxication has been reported when people drank large amounts much faster than healthy kidneys can clear, which is at most about 0.7 to 1.0 liters (about 24 to 34 fl oz) an hour. If you have a fluid limit from your doctor, follow it.
Do other drinks count toward the 8 glasses?
Yes. The NHS counts tea, coffee, lower-fat milk and lower-sugar drinks toward your daily intake, and Valtin's review noted that caffeinated drinks may be counted toward the daily total.
Should I drink 8 glasses even if I'm not thirsty?
For most healthy adults in everyday conditions, NASEM says thirst is a good guide. Older adults, whose thirst can weaken, and anyone who is active or in the heat for long periods may need to drink more deliberately.
Sources
- Valtin (2002). "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"?
- Maroz et al. (2012). Nonobstructive hydronephrosis due to social polydipsia: a case report
- National Academies (2004). Report Sets Dietary Intake Levels for Water, Salt, and Potassium to Maintain Health and Reduce Chronic Disease Risk
- Institute of Medicine, National Academies (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, chapter 4: Water
- European Food Safety Authority (2010). Scientific Opinion on Dietary Reference Values for water
- NHS (accessed 2026). Water, drinks and your health
This article is general information, not medical advice. If you have a health condition or your doctor has given you a fluid limit, follow their advice.
