How much water should I drink a day? The real numbers
Short answer: for most healthy adults, how much water you should drink comes to between 2 and 3.7 litres of total water a day, about four fifths of it from drinks. The exact figure depends on your size, your activity and your climate.
What health bodies actually publish
There is no single official number, but two large reviews are the ones most guidance is built on. Both give an adequate intake of total water: everything you drink plus the water in food.
| Source | Men, total water per day | Women, total water per day |
|---|---|---|
| US National Academies (Institute of Medicine), 2004 | 3.7 L (about 125 US fl oz) | 2.7 L (about 91 US fl oz) |
| European Food Safety Authority, 2010 | 2.5 L (about 85 US fl oz) | 2.0 L (about 68 US fl oz) |
The National Academies estimate that around 80 percent of total water comes from drinks and 20 percent from food. That puts the drinks part at roughly 3.0 L (101 oz) for men and 2.2 L (74 oz) for women. The same report sets 3.0 L of total water during pregnancy and 3.8 L while breastfeeding.
Where 8 glasses a day comes from
Eight 8-ounce glasses is 64 oz, about 1.9 L. In 2002 the physiologist Heinz Valtin looked for the science behind the rule and found none. It is not wrong as a habit, and for many women it lands near the drinks share above, but it is a rule of thumb, not a prescription.
When you need more
- Heat and humidity. You lose more through sweat without noticing it.
- Exercise. Drink before, during and after, more for long or hot sessions.
- Illness. Fever, vomiting and diarrhoea all raise what you lose.
- Pregnancy and breastfeeding. See the figures above.
Some people need less, or a fixed limit: people with heart failure or certain kidney conditions are often told to restrict fluids. If a doctor has given you a number, that number wins.
A simple way to hit your number
Pick a starting goal from the table, then watch two things: thirst, and the colour of your urine (pale straw is the usual sign you are fine). Spread drinks through the day instead of catching up at night. WaterDrop does the counting: one tap per glass, a goal in ounces or millilitres, and reminders that stop once you get there.
Questions people ask
Does coffee count toward my water intake?
Yes. The published intakes count water from all drinks and food. Coffee and tea still leave you with a net gain of water; WaterDrop counts them at their own rate.
How much water should I drink a day in ounces?
The US National Academies' adequate intake is about 125 oz of total water a day for men and about 91 oz for women, from all drinks and food. Roughly 101 oz and 74 oz of that comes from drinks.
Is 8 glasses of water a day right?
It is a rule of thumb, not a clinical number. A 2002 review by Heinz Valtin found no scientific study behind it. It lands close to the drinks share of the published intakes for many women, and short for many men.
Sources
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2004.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal 2010;8(3):1459.
- Valtin H. "Drink at least eight glasses of water a day." Really? Am J Physiol Regul Integr Comp Physiol 2002;283:R993.
This guide is general information, not medical advice.
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