Last reviewed: July 2026

Water: how much do you actually need?

There is no evidence-based universal rule that everyone must drink two litres of plain water a day. EFSA's adequate intake for total water is about 2.0 litres daily for adult women and 2.5 litres for adult men, and that total counts every source: tap water, coffee, tea, milk, soup and the water inside food. Food alone typically supplies 20 to 30 percent of it, because foods like cucumber and watermelon are over 90 percent water by weight. For most healthy people with normal kidneys, thirst is a reliable guide, and the honest answer to "how much should I drink" is: enough that you are rarely thirsty and your urine is pale.

Where the two-litre rule came from

The most cited origin is a 1945 US Food and Nutrition Board note suggesting roughly 1 millilitre of water per kilocalorie eaten, which lands near 2.5 litres for a typical adult. The sentence immediately after pointed out that most of that quantity is already present in prepared foods. That second sentence got dropped, the number became a drinking target, and bottled-water marketing did the rest. Reviews looking for trial evidence that healthy adults benefit from drinking beyond thirst have repeatedly come up empty.

What official bodies actually recommend

EFSA sets adequate intakes for total water, not for glasses of the plain stuff: about 2.0 litres a day for women and 2.5 litres for men, assuming moderate activity and a temperate climate. These are population averages, not personal prescriptions, and they scale with body size, sweat losses, heat and illness. Someone cycling to work in a Dutch August needs meaningfully more than the same person in February, and no fixed number captures that. Thirst does, which is precisely what it evolved for.

Your food is already a drink

Water content is the single most under-appreciated number on a food. It is also the reason two foods with identical calories can feel completely different, the mechanism covered in the energy density explainer. A few real examples from the database:

Someone eating plenty of vegetables, fruit and soup can hit adequate total water on noticeably less deliberate drinking than someone living on bread, crackers and cheese. Neither is doing it wrong; they simply arrive at the same total by different routes.

Does coffee dehydrate you?

Not at habitual intakes. Caffeine has a mild diuretic effect, but a cup of coffee delivers far more fluid than the extra urine it prompts, and controlled comparisons in regular coffee drinkers found no difference in hydration status between coffee and an equal volume of water. Alcohol is the genuine exception: it suppresses vasopressin and produces net fluid loss, which is a large part of why hangovers feel the way they do. Tea, milk and soft drinks all count normally, though the sugar in some of them is a separate question covered in the added sugar explainer.

When you genuinely need to drink deliberately

• Heat and hard exercise: sweat rates of 0.5 to 2.0 litres per hour are normal, and thirst lags behind losses during prolonged effort. Drink to a plan, not just to thirst, in sessions over about 90 minutes.
• Illness with fever, vomiting or diarrhoea: losses climb fast and appetite for fluid falls at the same time.
• Older age: thirst sensation genuinely blunts with age, which is one of the few situations where scheduled drinking beats waiting for the signal.
• Kidney stones: higher fluid intake is one of the better-evidenced preventive measures for people who form them.
• Certain medications and conditions: diuretics, lithium and some kidney conditions change the calculation. That is a conversation with your doctor, not with an article.

The overhydration edge case

Water is not free of risk at extremes. Healthy kidneys clear somewhere in the region of 0.8 to 1.0 litre per hour, and drinking well beyond that for a sustained period can dilute blood sodium into dangerous territory. Documented cases cluster around marathon runners who over-drink during races and occasional water-drinking contests. This is not a reason for anyone to be anxious about a normal day, but it does retire the idea that more water is automatically better.

A practical approach

Keep water available, drink when thirsty, drink extra around heat and exercise, and use urine colour as the feedback loop rather than a bottle with hour markings printed on it. If you are trying to cut calories, water and unsweetened drinks are also the cheapest single swap available: replacing a daily 330 ml sugary soft drink with water removes roughly 140 kcal a day without touching a single meal. Compare any two drinks you are choosing between in the comparison tool and the numbers make that case better than advice does.

Bottom line

Aim for around 2.0 to 2.5 litres of total water a day if you want a number, then stop counting. Food supplies a fifth to a third of it, coffee and tea count, alcohol does not, and thirst handles the rest for most healthy adults. The people who need to plan their intake are athletes, the ill, the elderly and stone formers, not the average person carrying a two-litre bottle out of guilt.