How Much Water Should You Drink a Day? What the Official Guidelines Say

Most healthy adults need roughly 2 to 3.7 litres of total water a day, but that includes the water in food and in drinks such as tea, coffee and milk, not just plain water. The famous “eight glasses a day” rule is a reasonable reminder rather than a scientific requirement. How much you personally need depends on your size, activity, climate, health and diet. This guide explains what the official guidelines actually say, how to tell whether you are drinking enough and when you need more, or less.

How Much Water Should You Drink a Day?

In the United States, the National Academies of Sciences, Engineering, and Medicine set what it calls an Adequate Intake for total water. According to its Dietary Reference Intakes, that is about:

• Men (19 and over): 3.7 litres (about 125 fluid ounces) of total water a day
• Women (19 and over): 2.7 litres (about 91 fluid ounces) a day
• Pregnancy: about 3.0 litres a day
• Breastfeeding: about 3.8 litres a day

The European Food Safety Authority (EFSA) sets lower reference values: 2.5 litres a day for men and 2.0 litres for women, again from all sources. The difference reflects different methods rather than a disagreement about health; both bodies stress that these are averages for healthy people in temperate climates.

Crucially, the US figures refer to total water. According to the National Academies, around 20% of daily water typically comes from food, and the rest from beverages. In practice, a man would drink about 3 litres (roughly 13 cups) and a woman about 2.2 litres (around 9 cups) of fluids a day to meet those reference values.

Is the “Eight Glasses a Day” Rule a Myth?

Partly. No major health body bases its guidance on precisely eight 8-ounce glasses of water. A widely cited review by physiologist Heinz Valtin, published in the American Journal of Physiology in 2002, found no scientific evidence for the “8 x 8” rule for healthy adults in temperate climates who are not exercising heavily.

A large study published in the journal Science in 2022, which measured water turnover in more than 5,600 people across 26 countries, found that needs vary widely with body size, physical activity, age, climate and economic development. The authors concluded that recommending eight glasses for everyone is not supported, as some people need less and others considerably more. Still, the UK’s NHS suggests six to eight glasses of fluid a day as practical everyday guidance, which lines up reasonably well with the reference values once food is included.

Does Coffee or Tea Count Towards Your Water Intake?

Yes. Caffeine has a mild diuretic effect, but research suggests that moderate amounts of coffee and tea hydrate you nearly as well as water for regular drinkers. Milk, sparkling water, herbal tea and even soup all count. The NHS notes that lower-fat milk and sugar-free drinks, including tea and coffee, count towards your fluid intake.

Sugary drinks and fruit juice also provide water, but they add calories and sugar, so they are better kept small. Alcohol is different: it suppresses a hormone that helps your kidneys hold on to water, so it is not a good way to hydrate.

How Can You Tell If You Are Drinking Enough?

For most healthy people, thirst is a reliable guide. Two other simple checks help:

• Urine colour: pale straw-coloured urine usually means you are well hydrated, while dark yellow or amber urine suggests you need more fluid
• Frequency: most people urinate around four to seven times in 24 hours, although this varies
• Symptoms of mild dehydration: thirst, a dry mouth, headache, tiredness and dizziness

Some vitamins and foods change urine colour, so treat the colour test as a rough guide rather than a precise measurement.

When Do You Need More Water?

• Exercise: you lose water through sweat; the American College of Sports Medicine advises drinking before, during and after prolonged exercise and weighing yourself before and after hard sessions to estimate losses
• Heat and humidity: hot weather can greatly increase sweat losses, even at rest
• High altitude: breathing faster in thin, dry air increases water loss
• Illness: fever, vomiting and diarrhoea can dehydrate you quickly, and oral rehydration solutions may help
• Pregnancy and breastfeeding: needs rise, as the figures above show
• High-protein or high-fibre diets: your kidneys and gut need water to process them; see our guide on how much protein per day

Who Is Most at Risk of Dehydration?

Older adults are particularly vulnerable because the sense of thirst tends to weaken with age, and some medications, such as diuretics, increase fluid loss. Babies and young children also dehydrate quickly because of their size. People who work outdoors or in hot environments, athletes and people with conditions such as diabetes should pay extra attention.

Can You Drink Too Much Water?

Yes, although it is uncommon in everyday life. Drinking very large amounts in a short time can dilute sodium in the blood, a condition called hyponatraemia (hyponatremia in US spelling). Symptoms include nausea, headache, confusion and, in severe cases, seizures. It is most often seen in endurance athletes who drink far more than they sweat, and in people with certain medical conditions.

Healthy kidneys can process a lot of water, but not unlimited amounts at once. Drinking to thirst during long events, rather than forcing fluids on a schedule, is the usual advice. People with heart failure or kidney disease may be told to restrict fluids, so follow your doctor’s guidance if that applies to you.

Which Foods Help Keep You Hydrated?

Fruit and vegetables are mostly water. Cucumber, lettuce, celery, tomatoes, watermelon, strawberries and oranges are all around 90% water or more. Soups, yoghurt and porridge also contribute. A diet rich in fruit and vegetables can make a noticeable difference to your total intake, which is one reason the official figures include food.

Simple Ways to Drink More Water

• Keep a refillable bottle with you and refill it at set times, such as each meal
• Drink a glass of water when you wake up and with every meal
• Add flavour with slices of lemon, cucumber or mint, or choose sparkling water
• Eat more water-rich fruit and vegetables
• Set phone reminders if you tend to forget during busy workdays
• Drink more before, during and after exercise and on hot days

Good hydration supports energy and concentration, but it is only one part of healthy living. For other everyday nutrition questions, see our guides on vitamin D and intermittent fasting.

Key Takeaways

US guidance puts total water intake at about 3.7 litres a day for men and 2.7 litres for women, including roughly 20% from food; European guidance is a little lower. Most people can meet their needs by drinking when thirsty, having a drink with each meal and checking that their urine stays pale. Drink more in hot weather, during exercise and when you are ill, and ask your doctor about the right amount if you have a heart, kidney or other medical condition. This article is general information, not medical advice.

Featured image: Glass of water ouside, by Kurt Kaiser / Wikimedia Commons, released under CC0. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for water, potassium, sodium, chloride, and sulfate (2005).
• European Food Safety Authority — Scientific opinion on dietary reference values for water (2010).
• Valtin H, American Journal of Physiology — “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”? (2002).
• Yamada Y et al., Science — Variation in human water turnover associated with environmental and lifestyle factors (November 2022).
• NHS — Water, drinks and your health (accessed 28 October 2026).
• US Centers for Disease Control and Prevention — About water and healthier drinks (accessed 28 October 2026).