Do you really need 8 glasses of water a day?
Not exactly. The "8×8" rule (eight 8-ounce glasses, about 2 litres) has no study behind it. But it's not far off for many adults, and it's a better habit than drinking nothing.
Updated October 17, 2026
It depends on your size
A 70 kg (154 lb) adult who mostly sits needs about 9 glasses (2.3 L). Smaller people need fewer, bigger or active people need more.
Where the 8 glasses rule came from
Nobody knows for sure. In 2002 the kidney physiologist Heinz Valtin went looking for the science behind "8×8" and found none. The closest source he found was a 1945 US recommendation of about 2.5 litres a day, which went on to say that most of it is already contained in food. That second sentence got lost.
What the guidelines actually say
| Guideline | Women | Men |
|---|---|---|
| US National Academies (from drinks) | ≈ 2.2 L · 9 glasses | ≈ 3.0 L · 12 glasses |
| European Food Safety Authority (drinks and food) | 2.0 L · 8 glasses | 2.5 L · 10 glasses |
So 8 glasses is about right for an average woman, and a bit low for an average man. The "2 litres a day" version of the rule is the same idea in metric.
How many glasses for your weight
| Weight | Mostly sitting | Very active |
|---|---|---|
| 50 kg (110 lb) | 7 glasses | 9 glasses |
| 60 kg (132 lb) | 8 glasses | 11 glasses |
| 70 kg (154 lb) | 9 glasses | 12 glasses |
| 80 kg (176 lb) | 11 glasses | 13 glasses |
| 90 kg (198 lb) | 12 glasses | 15 glasses |
| 100 kg (220 lb) | 13 glasses | 16 glasses |
Or use the calculator for your exact number.
Do the glasses have to be water?
No. Tea, coffee, milk and other drinks count, and so does the water in food, which is about a fifth of most people's intake. A 2014 study found that four cups of coffee a day kept people as hydrated as the same amount of water.
The real problem with 8 glasses is remembering. Tie them to things you already do: one on waking, one with each meal, one when you sit down to work.
Questions people ask
Is 2 litres of water a day enough?
For many adults, yes. People who are larger, more active or in the heat need more. Lighter people may need a little less.
How many ounces is 8 glasses of water?
64 fl oz, or about 1.9 litres.
How many glasses of water should a woman drink?
The US National Academies' figure works out to about 9 glasses from drinks for women, and about 12 for men. Your weight and activity change this.
Sources
- Valtin: "Drink at least eight glasses of water a day." Really? (Am J Physiol, 2002)
- National Academies of Sciences: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005)
- EFSA Panel on Dietetic Products: Scientific Opinion on Dietary Reference Values for water (2010)
- Killer et al.: No evidence of dehydration with moderate daily coffee intake (PLoS One, 2014)
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