How Much Water Should You Drink a Day, and How Much Is Too Much?

The NHS says aim for 6 to 8 cups or glasses of fluid a day, and water, lower-fat milk, tea and coffee all count; you may need more if you're pregnant or breastfeeding, in heat, active for long periods or ill. Too much is drinking well beyond thirst for hours, which can dilute the sodium in your blood.^{1, 2} The eight-glasses rule has no study behind it, and the NHS check is pee that's "a clear pale yellow colour".^{1, 3} This page is part of the Wellgard electrolyte guide.

Evidence at a glance

• Studies: the daily figures come from intake surveys, not trials; then two drink trials (72 and 50 men) and two race cohorts (488 and 669 runners tested).⁴
• Largest trial: 72 men, 13 drinks compared with still water over 4 hours; funded by a hydration research institute whose advisory board the lead author chaired.⁵
• Strongest measured outcome: after a litre, a rehydration solution and milk left about 300 g less urine than water over 4 hours.⁵
• Not tested: no study has set an everyday upper limit for healthy adults; the over-drinking data come from races.

Sources checked on 13 October 2026.

How much water should you drink a day?

About 6 to 8 cups or glasses of fluid, which the NHS calls "just a guide", with pale pee as the check. The figure is the government's, in the Eatwell Guide and on the NHS water page.^{1, 2} There is no separate figure for women or men, and none in litres.

The European Food Safety Authority does give litres. Its adequate intakes are 2.0 litres of total water a day for women and 2.5 for men, counting water from food as well as drinks, at moderate temperatures and moderate activity.⁴ Neither body sets an adult figure per kilogram, so the table below goes by situation, not weight.

The famous rule, eight 8-ounce glasses a day, was tested in a 2002 review: "No scientific studies were found in support of 8 x 8."³ Intake surveys of thousands of healthy adults suggested such amounts are not needed by sedentary people in a temperate climate; vigorous work, exercise and heat call for more.³

What counts towards your daily fluid?

Most drinks, and some food. In the NHS words, "Water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count".¹ Fruit juice and smoothies count too, kept to 150ml a day between them because of their sugar, and EFSA's figures include the water in food.^{2, 4}

The best measure is the 13-drink hydration index trial, funded by a hydration research institute whose advisory board the lead author chaired: 72 men drank a litre of water or other drinks and their urine was collected for four hours. Water left 1,337 g of urine; an oral rehydration solution left 1,038 g, full-fat milk 1,052 g and skimmed milk 1,049 g. Cola, diet cola, hot and iced tea, coffee, orange juice, sparkling water and a sports drink were no different from water.⁵ On the days that cost salt, a drink with enough sodium keeps more of what you drink.

Coffee counts too: in a three-day crossover trial in 50 men used to 3 to 6 cups a day, funded by a coffee research institute with one author at a drinks company, four 200 ml cups a day left total body water at 51.5 kg against 51.4 kg on water.⁶ While you are treating dehydration, the NHS advice changes: "Avoid caffeinated drinks and alcohol as both will dehydrate you."⁷

When do you need more water?

The NHS names four times: pregnant or breastfeeding, a hot environment, being physically active for long periods, and being ill or recovering from illness.¹ EFSA adds about 300 ml a day in pregnancy and about 700 ml when breastfeeding.⁴

A loss of 2% of body weight or more is where endurance performance measurably falls, while the effect on thinking "appears small".⁸ The 2007 sports-medicine position stand says to estimate your sweat rate by weighing yourself before and after exercise, and that drinks with electrolytes and carbohydrate "can provide benefits over water alone under certain circumstances".⁹

Being sick or having diarrhoea is different: start with small sips, and a pharmacist can recommend an oral rehydration solution to put back the sugar, salts and minerals lost.⁷ Related pages: water on creatine and headaches on tirzepatide.

How much water is too much?

There is no everyday litre limit; the harm documented in studies comes from drinking well beyond thirst for hours during long events, until the sodium in your blood is diluted. That is hyponatraemia. At the 2002 Boston Marathon, 13% of 488 runners tested finished with blood sodium at or below 135 mmol/L and 0.6% at or below 120. Gaining weight during the race (odds ratio 4.2), a finish over four hours (odds ratio 7.4) and extremes of body mass index were the risk factors; the type of fluid drunk was not.¹⁰

Over five years of a 161 km ultramarathon, 15.1% of 669 finishers tested were hyponatraemic, from 4.6% to 51% by year and more in hot years, and 35.6% of them had lost more than 3% of their body weight. The authors called "excessive sodium supplementation and aggressive fluid ingestion beyond the dictates of thirst" ill advised.¹¹ A 2017 review names drinking beyond thirst, with a hormone that makes the kidneys hold water, as the commonest causes.¹² Drink to thirst. Confusion or a fit after drinking heavily is a 999 call. Sodium on long runs and hyponatraemia in long events are covered in the runners guide.

Can drinking water lower your blood pressure, or raise it?

None of the studies on this page measured blood pressure, so we make no claim either way. The NHS does list dizziness on standing that does not go away as a 111 sign of dehydration.⁷ The minerals tied to blood pressure are salt and potassium (see how much salt a day and what potassium does); potassium, at 15% of its reference intake or more, contributes to the maintenance of normal blood pressure.

What does hydration actually mean?

Having the body water your cells and blood need, with thirst and the kidneys keeping it level. Dehydration "means your body loses more fluids than you take in"; the signs of dehydration have their own page.⁷ The 2002 review stressed how precisely a healthy body balances its water, which is why thirst and pee colour, not a fixed number, are the everyday guide.³

What we don't know

• The daily figures come from what healthy people were observed to drink, not from trials comparing amounts; a 2010 review found no long-term randomised trials of water intake.^{4, 13}
• The 13-drink trial tested rested, already-hydrated men only, and no sugar-free electrolyte sachet was among its drinks.⁵
• The coffee trial tested men used to 3 to 6 cups a day; people who rarely drink coffee were not studied.⁶

Frequently asked questions

Is it bad to drink 4 litres of water a day?

It's more than most people drink: alongside its adequate intakes, EFSA lists 95th-percentile total intakes of 3.1 litres a day for women and 4.0 for men, food included.⁴ The harm recorded in the studies came from drinking beyond thirst for hours during races, not from a large intake spread across a day.^{10, 11} If a doctor has set you a fluid limit, follow that.

Can drinking water kill you?

Rarely, yes. Hyponatraemia has caused deaths in marathon runners, and 0.6% of the Boston runners tested reached the critical level of 120 mmol/L or below.¹⁰ It takes hours of drinking beyond thirst, not a normal day.

Do electrolytes make you pee more or less?

Less, when the drink carries enough salts. In the 13-drink trial, funded by a hydration research institute whose advisory board the lead author chaired, an oral rehydration solution left 1,038 g of urine over four hours against 1,337 g after water, and the sports drink was no different from water. No sugar-free sachet was tested.⁵

Is sparkling water as hydrating as still?

Yes. Sparkling water was no different from still water in the 13-drink trial, and the NHS suggests it if you don't like the taste of plain water.^{5, 1}

Sources

1. NHS. Water, drinks and hydration. Page last reviewed 17 May 2023 (review overdue); checked 13 October 2026. nhs.uk
2. NHS. The Eatwell Guide. Page last reviewed 29 November 2022 (review overdue); checked 13 October 2026. nhs.uk
3. Valtin H. Review of the evidence for "8 x 8". American Journal of Physiology, 2002. PubMed 12376390.
4. EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary Reference Values for water, set from population intake data. EFSA Journal, 2010. doi.org/10.2903/j.efsa.2010.1459.
5. Maughan RJ, et al. Randomised crossover trial, 72 men, 4 hours per drink; funded by a hydration research institute whose advisory board the lead author chaired. American Journal of Clinical Nutrition, 2016. PubMed 26702122.
6. Killer SC, et al. Crossover trial, 50 men, two 3-day arms; funded by a coffee research institute, one author employed by a drinks company. PLoS One, 2014. PubMed 24416202.
7. NHS. Dehydration. Page last reviewed 1 May 2026; checked 13 October 2026. nhs.uk
8. Cheuvront SN, Kenefick RW. Review of dehydration and performance. Comprehensive Physiology, 2014. PubMed 24692140.
9. Sawka MN, et al. Position stand on exercise and fluid replacement. Medicine and Science in Sports and Exercise, 2007. PubMed 17277604.
10. Almond CS, et al. Observational cohort, 488 marathon runners tested. New England Journal of Medicine, 2005. PubMed 15829535.
11. Hoffman MD, et al. Observational, 887 ultramarathon finishers, 669 with sodium measured, 5 years. Medicine and Science in Sports and Exercise, 2013. PubMed 23135369.
12. Hew-Butler T, et al. Review of exercise-associated hyponatraemia. Frontiers in Medicine, 2017. PubMed 28316971.
13. Popkin BM, et al. Review of water, hydration and health. Nutrition Reviews, 2010. PubMed 20646222.

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