How much water should you drink every day?

You have probably heard it a thousand times: drink eight glasses of water a day. It is simple, memorable, and almost entirely unsupported by science. The truth is that how much water your body needs depends on who you are, what you do, and where you live — and for most people, the real figure is quite different from eight glasses.

This guide explains where the recommendation actually comes from, what the evidence says, and how to calculate a personalised daily target that makes sense for you.

Where did "eight glasses a day" come from?

The eight-glasses rule is one of the most persistent myths in popular health advice. Its most likely origin is a 1945 US National Research Council recommendation that suggested adults consume around 2.5 litres of water per day — but the very next sentence, almost always omitted when this figure is repeated, noted that most of that quantity was already contained in the food people eat.

A 2002 review published in the American Journal of Physiology examined the scientific basis for the "8 × 8" rule — eight 8-ounce glasses per day — and found no evidence to support it. The author concluded the recommendation had no scientific basis and was likely perpetuated through repetition rather than research.

How much water do you actually need?

The most widely used method for estimating individual water needs is based on body weight. A commonly cited guideline from sports medicine and nutrition research suggests approximately 35 ml of water per kilogram of body weight per day as a baseline for a sedentary adult in a temperate climate.

From there, additional amounts are added based on activity level and environment:

So a 70 kg person who exercises moderately and lives in a temperate climate would need approximately:

Use our water intake calculator to get a personalised figure based on your own weight, activity level, and climate.

Does it have to be plain water?

No — and this is another area where the eight-glasses rule misleads people. Your body does not distinguish between water from a glass and water from food or other drinks. All fluids count toward your daily total, and a significant proportion of your intake comes from food.

Research suggests that roughly 20 to 30 per cent of daily water intake comes from food for most people. Fruit and vegetables in particular have very high water content — cucumber and lettuce are over 95% water, while most fruits sit between 80% and 90%.

Tea and coffee count too, despite the persistent myth that caffeine cancels out their hydrating effect. While caffeine is a mild diuretic, the fluid in caffeinated drinks still contributes positively to hydration overall. A 2014 study found no significant difference in hydration status between people who drank coffee and those who drank water.

Signs you are not drinking enough

Rather than counting glasses, many hydration experts suggest paying attention to your body's signals. The most reliable indicators of dehydration are:

• Urine colour — pale straw yellow indicates good hydration; dark yellow or amber suggests you need to drink more
• Thirst — a reliable signal in healthy adults, though less reliable in older people whose thirst mechanism becomes less sensitive with age
• Headache or difficulty concentrating — even mild dehydration (1–2% of body weight) can impair cognitive performance
• Dry mouth or lips — an early sign that your fluid intake is running behind your needs
• Passing urine less often than usual — a change from your own normal pattern is more informative than any particular number

Can you drink too much water?

Yes, though it is rare in healthy people going about their daily lives. Overhydration — drinking so much water that the sodium concentration in the blood drops dangerously low — is a condition called hyponatraemia. It is most commonly seen in endurance athletes who drink excessive quantities of plain water during long events without replacing electrolytes.

For most people the practical risk is not overhydration but under-hydration, particularly in hot weather, during illness, or when exercising for extended periods.

Practical tips for hitting your daily target

• Start with a large glass first thing in the morning — you lose fluid overnight through breathing and will often wake mildly dehydrated
• Carry a reusable bottle — knowing its volume makes it easy to track your intake without counting individual glasses
• Drink a glass before each meal — three meals means three easy glasses ticked off, and it can also help with portion control
• Set hourly reminders if you work at a desk and tend to forget; many people find they drink most of their fluid in the evening when it is too late to be useful
• Increase intake proactively in hot weather — do not wait until you feel thirsty; by that point you are already mildly dehydrated

How hydration affects your health

Staying well hydrated has measurable effects across a wide range of body functions. Water plays a role in regulating body temperature, transporting nutrients, lubricating joints, and supporting kidney function. Even mild dehydration has been shown in research to reduce physical performance, impair short-term memory, and increase feelings of fatigue and anxiety.

Low fluid intake is associated with a higher risk of kidney stones and of constipation.

The bottom line

Eight glasses a day is a rough average, not a prescription. A weight-based estimate of total water, adjusted for exercise and heat, gives a sense of scale — and urine colour is the more useful day-to-day guide.

For a 70 kg person who exercises moderately, the weight-based estimate of total water is closer to 3 litres than 2 litres — but that is total water, and roughly a quarter of it normally comes from food. The NHS figure for drinks alone is 6 to 8 cups or glasses a day. Neither number is a target to hit; urine colour is the more useful day-to-day guide.

Sources and further reading

• National Health Service. Water, drinks and your health. NHS, 2024. nhs.uk/live-well/eat-well/food-guidelines-and-food-labels/water-drinks-nutrition — the 6 to 8 cups or glasses a day figure, what counts towards it, and the pale-yellow urine check. Checked 18 October 2026.
• EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010;8(3):1459. — total water reference values and the share that comes from food.
• Valtin H. “Drink at least eight glasses of water a day.” Really? American Journal of Physiology, 2002;283(5):R993–R1004. pubmed.ncbi.nlm.nih.gov/12376390 — where the eight-glasses rule came from, and the absence of evidence for it. Record checked 19 October 2026.
• Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake. PLoS ONE, 2014;9(1):e84154. pubmed.ncbi.nlm.nih.gov/24416202 — why tea and coffee count towards fluid intake. Record checked 19 October 2026.
• Benton D, Young HA. Do small differences in hydration status affect mood and mental performance? Nutrition Reviews, 2015;73(suppl 2):83–96. pubmed.ncbi.nlm.nih.gov/26290294 Record checked 19 October 2026.
• Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 2015;25(4):303–320. pubmed.ncbi.nlm.nih.gov/26102445 — the risks of drinking too much, and who they affect. Record checked 19 October 2026.