How much water should you drink a day? Official guidance and simple reminders

What the official guidance says

Three sources come up most often, and their numbers look different because they measure different things.

NHS (UK). Aim for 6 to 8 cups or glasses of fluid a day. The NHS presents this as a general guide, not a strict target. Water, lower-fat milk and sugar-free drinks, including tea and coffee, all count.

US National Academies (2004). Their report on water set “adequate intakes” based on what healthy, well-hydrated people actually consume: about 2.7 litres of total water a day for women and 3.7 litres for men. Total water means drinks and the water in food. About 80% of it came from drinks and 20% from food. The report’s headline message was that most people get enough “by letting their thirst guide them”.

EFSA (2010). The European panel set adequate intakes of total water, again including food, at the levels below. They apply only to moderate temperatures and moderate physical activity.

The US and European figures differ because the two panels used different data and methods. Either way, none of them is a personal prescription.

The water you eat

Food hydrates more than people think. According to EFSA, water content is:

• over 80% in fruit and vegetables;
• about 90% in milk;
• 40–70% in hot meals;
• under 40% in baked goods.

A day of fruit, vegetables, soups and stews brings far more water than a day of sandwiches and crackers. That’s one more reason not to read “2 litres” literally: 2 litres of total water isn’t 2 litres from bottles.

The rough maths, using EFSA’s figures and the 80/20 split: a woman would drink about 1.6 litres (80% of 2.0) and a man about 2.0 litres (80% of 2.5) in ordinary conditions. The US figures give about 2.2 litres (roughly 9 cups) for women and 3.0 litres (roughly 13 cups) for men from drinks. These are averages that shift with your diet, the weather and how active you are.

What counts as a drink?

For the NHS, water, lower-fat milk and sugar-free drinks, including tea and coffee, all count. Physiologist Heinz Valtin’s 2002 review reached the same conclusion: caffeinated drinks can count towards the daily total.

That doesn’t make every drink equally good for you. The Dietary Guidelines for Americans, 2025–2030 say to choose water, still or sparkling, and unsweetened drinks. The NHS suggests limiting fruit juice and smoothies to 150 ml a day in total. If most of your fluid comes from soda or juice, our plan to cut down on sugar starts precisely with drinks.

When do you need more?

The standard figures assume mild weather and moderate activity. Several situations push needs up:

• Heat and exercise. EFSA notes that water losses from heat and physical work can reach about 8 litres a day, and that lost salts then need replacing too. The National Academies report that very active people continually exposed to hot weather often need 6 litres or more of total water a day.
• Illness. The NHS lists recovering from illness among the times you may need more fluid. For a baby, an older person or anyone frail who is being sick or has diarrhoea, get advice early from a doctor, pharmacist or NHS 111 in the UK.
• Pregnancy and breastfeeding: about 300 ml and 700 ml more a day, according to EFSA.
• Older age. EFSA points out that thirst weakens in older people, which puts them at greater risk of not drinking enough. Drinking at set times rather than waiting for thirst can help.

The opposite also applies: some conditions, particularly kidney and heart disease, sometimes require limiting fluids. If that’s you, follow your doctor’s instructions rather than general guidance.

The “8 glasses” myth, and the opposite risk

“Drink at least eight 8-ounce glasses of water a day” (about 1.9 litres) has been repeated for decades. In 2002, Dartmouth physiologist Heinz Valtin set out to find where it came from and what evidence supports it, in a review in the American Journal of Physiology. He found no scientific studies in support. Surveys of thousands of healthy adults suggested such amounts aren’t needed for people living a mostly sedentary life in a temperate climate, especially since food and all kinds of drinks count. He also stressed that larger intakes are justified for some diseases, hard physical work and hot climates.

Drinking far too much carries its own risk. EFSA explains that adult kidneys can clear at most about a litre of urine an hour. Beyond that, a very fast excess of water can dilute the sodium in your blood (hyponatraemia), which can be serious. It has been seen in endurance athletes drinking large volumes of plain water. EFSA set no universal upper limit, because it depends on the person and the setting.

A simple check from the NHS: drink enough during the day that your pee is a clear pale yellow.

Reminders that actually help

If you know you don’t drink enough, at your desk or in hot weather, the problem is rarely information. It’s forgetting. Two findings from habit research help:

1. Tie the glass of water to something you already do. “After my morning coffee, I drink a glass of water.” “Every time I stand up for a break, I refill my bottle.” This is habit stacking, and our morning routine guide starts with exactly that glass of water.
2. Be patient. In Phillippa Lally’s 2010 study, drinking habits such as having a bottle of water with lunch took a median of 59 days to become automatic. A reminder is a crutch for that period.

A water reminder app doesn’t need to be complicated: a nudge at times you choose and a quick way to log a glass. In Binome360, you set up recurring reminders in one sentence and can log what you drink in a few words. Your assistant drafts it; you confirm.

Frequently asked questions

Do I really need 8 glasses of water a day?

Not as a fixed rule. The NHS gives 6 to 8 glasses of fluid as a general guide, and tea, coffee and milk count. Valtin’s 2002 review found no study behind “8 x 8”. Your needs depend on your size, the weather, your activity and how much water your food provides.

Does coffee dehydrate you?

The NHS counts tea and coffee towards your daily fluid, and Valtin’s review came to the same view for caffeinated drinks. Water and unsweetened drinks are still the best default, as the US Dietary Guidelines put it.

Can you drink too much water?

Yes, in particular situations: drinking very large amounts of plain water in a short time, for example during endurance sport, can cause hyponatraemia. For a healthy adult drinking to thirst in daily life, the risk is low.

Should I drink even when I’m not thirsty?

For healthy adults in ordinary conditions, the National Academies say normal drinking and thirst are enough. It’s different for older people, whose thirst fades, and in heat or during exercise, when drinking regularly is safer.

In short

Official guidance puts total water at about 2 to 2.7 litres a day for women and 2.5 to 3.7 litres for men, food included, with roughly a fifth coming from what you eat; the NHS’s simple version is 6 to 8 glasses of fluid. Heat, exercise, illness, pregnancy and breastfeeding raise needs, and “8 glasses” has no scientific basis. First step: pick two fixed moments in your day and attach a glass of water to each.

Sources

• NHS, “Water, drinks and your health”, reviewed 2023: nhs.uk.
• National Academies of Sciences, Engineering, and Medicine, “Report Sets Dietary Intake Levels for Water, Salt, and Potassium to Maintain Health and Reduce Chronic Disease Risk”, 11 February 2004: nationalacademies.org.
• EFSA Panel on Dietetic Products, Nutrition and Allergies, “Scientific Opinion on Dietary Reference Values for water”, EFSA Journal, 8(3):1459, 2010: doi.org/10.2903/j.efsa.2010.1459.
• EFSA, Summary of Dietary Reference Values, version 4, September 2017, table 3: efsa.europa.eu.
• Heinz Valtin, “‘Drink at least eight glasses of water a day.’ Really? Is there scientific evidence for ‘8 x 8’?”, American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, 283, R993–R1004, 2002: doi.org/10.1152/ajpregu.00365.2002.
• U.S. Department of Health and Human Services and U.S. Department of Agriculture, Dietary Guidelines for Americans, 2025–2030, 2026: realfood.gov.
• Phillippa Lally et al., “How are habits formed: Modelling habit formation in the real world”, European Journal of Social Psychology, 40(6), 2010: doi.org/10.1002/ejsp.674.

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