Quick answer (TL;DR): There is no single water rule that fits everyone. The US National Academies set an adequate intake of about 2.7 litres of total water a day for adult women and 3.7 litres for men, counting all drinks and the water in food (food supplies roughly a fifth). These are averages for healthy people, not targets. Thirst and pale-yellow urine are reasonable guides, and you need more in hot weather, during exercise and when you are ill.
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How much water do you need each day?
The figures below are "adequate intakes" from the US National Academies. They describe what healthy, well-hydrated people in the United States and Canada typically take in. They are not minimums you must reach, and they include water from food as well as from every drink.
| Group | Total water per day (drinks and food) | Roughly from drinks, if a fifth comes from food |
|---|---|---|
| Adult women | about 2.7 litres | about 2.2 litres |
| Adult men | about 3.7 litres | about 3 litres |
| Pregnancy | about 3.0 litres | ask your doctor or midwife |
| Breastfeeding | about 3.8 litres | ask your doctor or midwife |
Your own need depends on body size, climate, activity and diet. In a hot, humid country you usually lose more water through sweat than the table assumes.
Is "8 glasses a day" a real rule?
No. It is a popular rule of thumb, not a recommendation from the major health bodies, and it ignores the water that comes from food, tea and other drinks. Eight glasses of about 240 ml each come to roughly 1.9 litres, which is less than the 2.2 to 3 litres of drinks suggested by the adequate intakes above. For many healthy adults it is roughly in the right range, so there is no harm in it, but there is no need to force it either.
How can you tell if you are drinking enough?
- Urine colour: pale yellow through most of the day usually means you are drinking enough. Dark yellow suggests you need more fluid. Some vitamins, foods and medicines change urine colour, so use it as a guide, not a test.
- Thirst: for most healthy adults, thirst is a good signal. Older adults can feel thirst less, so they may need to drink on a routine.
- Signs of dehydration: strong thirst, dark urine, headache, tiredness and dizziness. Mild dehydration can make you feel tired and less able to concentrate.
Do tea and coffee count as water?
Yes. Fluid from tea, coffee, milk and other drinks counts toward your total, and in usual amounts coffee does not cancel out its own water. Plain water or unsweetened tea is still a better everyday choice than sugary drinks, which add sugar and calories. Very large amounts of caffeine have other downsides, so keep it moderate.
Fruit, vegetables, soups and cooked rice also add to your total: cucumber and lettuce are about 95 percent water, watermelon about 91 percent, and cooked rice roughly two-thirds. See our post on eating more fruits and vegetables.
Which viral water myths should you ignore?
- "A glass before bed prevents strokes or heart attacks." I could not find good evidence for this. Drink if you are thirsty, but do not rely on a timed glass for protection.
- "A drink before a shower prevents high blood pressure." There is no good evidence for this either. Blood pressure needs regular checks and, if it is high, proper treatment.
- "Two glasses on waking switch on your organs." Your organs work without it. A morning drink is pleasant, but it is not a medicine.
Which water beliefs do I hear in villages and neighbourhoods?
From my community health work, these are beliefs I hear often, many of them from people I respect and who mean well. Here is what the evidence says about each.
"Everyone must drink 8 glasses a day."
This is a rule of thumb, not a requirement. Needs differ with body size, weather, work and health, and tea, food and other drinks count too (see above).
"Drinking water during or just after a meal dilutes the digestive juices."
There is no good evidence for this. The stomach adds its own fluid and makes more acid and enzymes as food arrives, and reviews have found that drinking with meals is unlikely to harm digestion in healthy people. The exception is people with acid reflux (GERD), who may feel better drinking between meals instead. Sugary drinks with meals are a different matter, because they add calories.
"Drinking water while standing causes arthritis or joint pain in the knees."
No. Arthritis comes from a mix of biological and environmental factors, such as age, injury, body weight and autoimmune disease, not from the posture in which you drink. Whatever your posture, sip rather than gulp to avoid choking.
"The more water you drink, the healthier you are."
Not so. Once your needs are met, extra water adds no benefit. Drinking very large volumes in a short time can dilute the sodium in your blood, and people with heart, kidney or liver disease may be advised to limit fluids.
"Cold water makes fat build up in the body."
I could not find good evidence for this. Body fat gain comes from taking in more energy than you use, not from the temperature of a drink. If very cold water bothers your throat or teeth, room-temperature water hydrates you just as well.
"Drink only when you feel thirsty."
For most healthy adults on most days, thirst is a good guide. But it is not enough in some situations. Older adults can feel thirst less, and in hot weather, during hard physical work or exercise, and during illness, it is wise to drink a little before you feel thirsty. Children also need adults to offer them drinks.
Does drinking enough water help prevent kidney stones?
Staying well hydrated is a standard piece of advice for people who have had kidney stones, because more fluid dilutes the urine. If you have had a stone, ask your doctor how much fluid is right for you.
When do you need more water than usual?
You may need more fluid in hot weather, during long or hard exercise, during pregnancy and breastfeeding, and when you have a fever, vomiting or diarrhoea. Drink to thirst, plus a little more in these situations.
For diarrhoea, the WHO advises oral rehydration salts (ORS), a mixture of clean water, salt and sugar that replaces the water and salts you lose. Plain water alone is not enough in a serious bout. See a health worker quickly if there is blood in the stool, if the person cannot drink, has sunken eyes or is unusually sleepy, or if diarrhoea does not improve within about three days. Children and older people can become dehydrated faster.
Can you drink too much water?
Yes, though it is rare. Drinking very large volumes in a short time can dilute the sodium in the blood (hyponatremia), which can cause headache, nausea, confusion and, in severe cases, seizures. This has mostly been reported in endurance events and drinking challenges. In daily life, drinking to thirst is safe for most people.
Is your drinking water safe?
How much you drink matters less if the supply is unsafe. In Bangladesh, many shallow tube wells have been found to contain arsenic. In an earlier national survey of 3,534 tube wells, 27 percent of shallow wells (under 150 metres) and 1 percent of deep wells (over 150 metres) exceeded the national limit of 50 micrograms per litre, while the WHO guideline is 10 micrograms per litre. A WHO Bulletin review advises that groundwater used for drinking should be tested for arsenic. If you do not know whether your tube well has been tested, ask your local health or water authority. Also remember that contaminated supplies are a major cause of diarrhoeal disease.
Who should check with a doctor about how much to drink?
- People with heart failure, kidney disease or liver disease, who may have been told to limit fluids.
- People taking water tablets (diuretics) or other long-term medicines.
- Pregnant or breastfeeding women who feel very thirsty, dizzy or unwell.
- Anyone with persistent excessive thirst or very frequent urination, which can be a sign of a medical problem such as diabetes.
Get urgent medical help for confusion, fainting, a rapid heartbeat, an inability to keep fluids down, or very little urine.
What are some practical hydration tips?
- Keep a bottle or glass within sight.
- Have a glass with each meal.
- Before and during exercise, drink to thirst, and a little more in the heat.
- Choose unsweetened tea or plain water over sugary drinks.
- When you are ill, take small, frequent sips instead of large amounts at once.
For more everyday ideas, see our nutrition tips.
Frequently Asked Questions
How do I know if I am dehydrated?
Common signs are strong thirst, dark yellow urine, headache, tiredness and dizziness. Pale-yellow urine usually means you are drinking enough. Seek medical help if symptoms are severe or you cannot keep fluids down.
Does coffee dehydrate you?
No, not in usual amounts. Coffee and tea contribute to your daily fluid intake. Water should still be your main drink, and very high caffeine intake has other downsides.
Is it bad to drink water during meals?
There is no good evidence that a moderate amount of water with meals harms digestion in healthy people.
Should I force myself to drink 8 glasses a day?
No. Drink regularly through the day, more when it is hot or you are active, and use thirst and urine colour as your guide. If a doctor has limited your fluids, follow their advice instead.
Sources
- Institute of Medicine (National Academies), Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, 2005
- Healthline: evidence-based review of drinking liquids with meals
- Versus Arthritis: myths about arthritis, including drinking water while standing
- World Health Organization: Diarrhoeal disease fact sheet
- National Hydrochemical Survey of Bangladesh: arsenic in tube well water
- Bulletin of the World Health Organization: Contamination of drinking-water by arsenic in Bangladesh
General information only, not medical, healthcare, or psychological advice. Check current medical guidelines with a certified physician or licensed healthcare provider before acting on anything published here. Never disregard professional medical advice or delay seeking it because of something you read on this site.
