How Much Water Do You Need? What the Evidence Says
What the National Academies, the NHS and research say about daily fluid needs, the 8-glasses rule, coffee and hydration, thirst, too little water and too much water.
Content last updated . This page has not been medically reviewed.
"Drink eight glasses of water a day" is one of the most repeated health rules. Where does it come from, and what do official bodies and studies actually say? This guide looks at daily fluid needs in plain terms: what counts, what changes the amount, how to tell if you are drinking too little, and why drinking far too much can also be dangerous. It gives information from official sources, not a personal prescription. For the drink itself (tap and bottled water, lead, fluoride) see the water page.
Read this first
- There is no single proven number of glasses. The National Academies' figure for total water is an "adequate intake" based on what healthy people in US surveys usually take in. It is not a requirement, because a wide range of intakes supports normal hydration. [1]
- That figure includes food. In the survey behind it, drinks supplied about 81 percent of total water and food about 19 percent. [1]
- Thirst and urine color are the practical guides. The National Academies say normal hydration is kept by thirst and normal drinking habits. The NHS says to drink enough that your urine is pale. [1] [2] [3]
- Needs go up in heat, with long or hard exercise, in illness with vomiting or diarrhea, and in pregnancy or breastfeeding. [1] [2]
- Too much, too fast, can be dangerous. Drinking more than the kidneys can clear can lower blood sodium, which can be a life-threatening emergency. MedlinePlus says to call your provider right away if symptoms appear; for severe symptoms such as seizures or confusion, use your local emergency number (911 in the US). [1] [4]
What the official numbers say
| Body | Figure | What it covers |
|---|---|---|
| National Academies (Institute of Medicine, 2005) | Adequate intake of total water: 3.7 liters a day for men and 2.7 liters for women aged 19 to 30. Fluids alone in the survey: about 3.0 liters (men) and 2.2 liters (women). [1] | All water from drinks and from food. Not a requirement. No upper limit was set because healthy people can excrete extra. [1] |
| NHS (UK, 2023) | "6 to 8 cups or glasses of fluid a day" as a general guide, and pale-yellow urine as a check. [2] | Water, lower-fat milk and lower-sugar drinks, including tea and coffee, count. [2] |
| Dietary Guidelines for Americans 2020-2025 | No fixed volume of water. The guidelines say the water in foods such as fruits, vegetables and soups adds to hydration and to total fluid intake. [10] | Whole diet |
These bodies do not agree on one number, and they measure different things (total water versus drinks). That is why we show them side by side rather than choosing one.
Where "8 glasses" came from
A physiologist (Valtin, 2002) searched for scientific support for the "8 x 8" advice and found none. His review notes that surveys of thousands of healthy adults in temperate climates suggest such large intakes are unnecessary, and that larger intakes are advisable in some diseases and in vigorous exercise or hot climates. The conclusion applies only to healthy, sedentary adults in temperate climates. [5] The rule is easy to remember, but we found no study in the sources we read that proves it.
Why your needs may be different from someone else's
A large 2022 study in Science measured water turnover (how much water the body takes in and replaces) with an isotope method in 5,604 people aged 8 days to 96 years from 23 countries. It found that age, body size and composition, physical activity, athletic status, pregnancy, socioeconomic status and the environment (latitude, altitude, air temperature, humidity) were all linked to turnover, and that people in lower-income countries had higher turnover. [7] The abstract we read does not give the figures in liters, so we do not quote them.
In practice:
- Heat and exercise. The National Academies report that physically active people and people in hot places need more total water, and that daily sweat losses in hot climates can exceed several liters. [1] The NHS says to drink extra after sweating from exercise. [2]
- Older adults. The National Academies note that older people concentrate urine less well, so they lose more water in urine; the NHS lists older adults among the groups at higher risk of dehydration. [1] [3]
- Illness. Vomiting, diarrhea and fever raise the risk of dehydration. [3] See Oral rehydration solutions.
- Pregnancy and breastfeeding. The NHS says needs rise. [2] We did not read an official figure for how much more.
Do coffee, tea and other drinks count?
The NHS says water, lower-fat milk and lower-sugar drinks, including tea and coffee, count toward your fluid. [2] A crossover trial of 50 men who usually drank 3 to 6 cups of coffee a day compared coffee (about 4 mg of caffeine per kg of body weight, in four 200 ml servings a day) with water for three days each. Total body water and 24-hour urine volume did not differ (about 2.4 liters either way), so coffee in moderation performed like water in these men. [6] It is one small trial in habitual coffee drinkers, and the abstract does not state who paid for it. Sugary drinks add calories; the NHS suggests keeping juice and smoothies to a small 150 ml glass a day. [2] See Caffeine and sugar in drinks.
Signs you may be drinking too little
The NHS lists thirst, headache, light-headedness, tiredness, dry mouth, lips and tongue, and dark yellow strong-smelling urine passed less often. In babies, look for a sunken soft spot, sunken eyes, few or no tears, fewer wet diapers than usual and drowsiness. [3] Babies, children and older adults are at higher risk, and so are people with diabetes, a high temperature, heavy sweating, heavy drinking, or who take water pills (diuretics) or rely on others for drinks. [3]
Get urgent medical help (in the UK, call 999 or go to A&E; in the US, call 911) for blue, grey, pale or blotchy skin, lips or tongue, cold skin, trouble breathing, confusion, or unusual sleepiness or difficulty waking. These can be signs of shock. [3]
Too much water
- How it happens. The National Academies report water toxicity after rapid drinking of large volumes that exceeds the kidneys' ability to excrete about 0.7 to 1.0 liter an hour. [1] A review of exercise-associated low blood sodium says overdrinking beyond thirst is among the most common causes. [9]
- Symptoms. MedlinePlus lists confusion, irritability, headache, fatigue, loss of appetite, muscle weakness or cramps, nausea, vomiting and seizures, and says a sharp drop in sodium can be life-threatening. It also lists causes other than drinking: burns, diarrhea, vomiting, sweating, water pills, heart failure, kidney disease and cirrhosis. [4]
- See Hyponatremia: low sodium, causes, symptoms and water intoxication.
What water has been tested for
| Goal | What the sources found | Strength of evidence | What was studied |
|---|---|---|---|
| Kidney stone recurrence | In a 5-year randomized trial of 199 people after a first calcium stone, stones recurred in 12 of 99 who raised water intake and 27 of 100 who did not. [8] | Promising but limited (one trial) | People with a first idiopathic calcium stone |
| Eating less, weight | A systematic review found energy intake 7.8 percent higher after sugar-sweetened drinks than after water at a meal; the authors said randomized trials are needed to confirm that water helps reduce energy intake. [11] | Mixed | Clinical trials, population and intervention studies |
| Hydration level and health | A 2025 Austrian study of 5,817 adults aged 40 to 70 linked normal hydration markers and good health with a median plain water intake of at least 1 liter a day. [12] | Insufficient evidence (observational only) | Adults aged 40 to 70 in one region |
| "Detox", skin glow, sharper thinking | Proper evidence not found in the sources we read. | No good evidence of benefit found | Not applicable |
Related pages
Frequently asked questions
Do I really need eight glasses of water a day?
A review of the evidence found no scientific support for the "8 x 8" rule, though it applies to healthy, sedentary adults in temperate climates and not to hot weather, hard exercise or some diseases. [5] The NHS suggests 6 to 8 cups or glasses of fluid a day as a general guide. [2]
Do coffee and tea count toward my water?
The NHS says tea and coffee count. [2] In a small trial of men who were regular coffee drinkers, moderate coffee gave the same total body water and urine volume as water. [6]
How can I tell if I am drinking enough?
The NHS says to drink enough during the day that your urine is pale yellow; dark yellow, strong-smelling urine is a sign of dehydration. [3]
Can you drink too much water?
Yes. The National Academies report toxicity after rapid drinking of large volumes beyond what the kidneys can excrete, about 0.7 to 1.0 liter an hour. [1] MedlinePlus says very low blood sodium can be life-threatening. [4]
Does drinking more water prevent kidney stones?
One 5-year randomized trial found fewer recurrences in people who raised their water intake after a first calcium stone. [8] It is a single trial, so ask a clinician about your own case.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Practical, low-risk habits
- Drink when thirsty and with meals; the National Academies say drinking with meals helps replace losses. [1]
- Look at your urine: pale yellow is the NHS check. [2] [3]
- Plan extra in heat or long exercise, and during illness with vomiting or diarrhea. [1] [3]
- If you have kidney, heart or liver disease or take water pills, ask your clinician about fluid. Some people are told to limit fluids; we did not read a source that gives limits. [4]
- Where the safety of tap water is in question, see the water page and follow local health advice.
References
- Institute of Medicine (National Academies). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. 2005. Read 2026-10-24 (first 100,000 characters of the chapter).
- NHS. Water, drinks and hydration. Last reviewed 17 May 2023. Read 2026-10-24.
- NHS. Dehydration. Reviewed 1 May 2026. Read 2026-10-24.
- MedlinePlus. Low blood sodium. Reviewed 2025-05-19. Read 2026-10-24.
- Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? Am J Physiol Regul Integr Comp Physiol 2002 (abstract read via Europe PMC). Read 2026-10-24.
- No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One 2014 (abstract read via Europe PMC). Read 2026-10-24.
- Variation in human water turnover associated with environmental and lifestyle factors. Science 2022 (abstract read via Europe PMC). Read 2026-10-24.
- Borghi L et al. Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. J Urol 1996 (abstract read via Europe PMC). Read 2026-10-24.
- Exercise-associated hyponatremia: 2017 update. Front Med 2017 (abstract read via Europe PMC; the abstract text we saw covers causes, not prevention advice). Read 2026-10-24.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020-2025. Read 2026-10-24.
- Daniels MC, Popkin BM. Impact of water intake on energy intake and weight status: a systematic review. Nutr Rev 2010 (abstract read via Europe PMC). Read 2026-10-24.
- Stookey JD et al. Hydration and health at ages 40-70 years in Salzburg Austria is associated with a median total water intake over 40 mL/kg including at least 1 L/d plain drinking water. Front Public Health 2025 (abstract summary read via Europe PMC). Read 2026-10-24.