Do You Really Need Eight Glasses of Water a Day?

Eight glasses a day is easy to remember, but hydration is more personal than a single number suggests. Your food, activity level, health and surroundings all influence how much fluid you actually need.

Originally published on January 13, 2020, this feature has been substantially updated with current hydration guidance, newer research and more practical advice.

Where Did the Eight-Glass Rule Come From?

The familiar advice to drink eight glasses of water a day has endured because it is simple, memorable and broadly sensible. What it is not, however, is a precise requirement that applies equally to every person.

Official reference figures are considerably broader. The US National Academies sets an adequate intake of approximately 3.7 litres of total water per day for adult men and 2.7 litres for adult women. Those figures include plain water, other beverages and the moisture contained in food—they are not instructions to drink that amount as additional bottled or tap water.

They are also population-level reference values, not personalised prescriptions. A smaller person spending a quiet day indoors will not necessarily need the same amount as someone exercising outside in summer.

Food and Other Drinks Count

Plain water remains one of the easiest and healthiest ways to stay hydrated, but it is not the body’s only source of fluid.

Tea, coffee, milk and sugar-free drinks all contribute to daily intake. So do foods such as fruit, vegetables, soups, yoghurt and cooked grains. National Academies data indicate that food supplied about 19% of total water intake in the population used to establish its reference figures.

That means a person who eats plenty of water-rich foods may obtain a meaningful amount of fluid without noticing it. Someone eating mostly dry, processed foods may need to rely more heavily on drinks.

The NHS uses six to eight cups or glasses of fluid as an everyday guide but emphasises that people may need more when they are physically active, pregnant, breastfeeding, ill or in a hot environment.

Why Your Needs Change

Daily fluid requirements shift with body size, age, activity, climate and health. A large international study involving 5,604 people across 23 countries found that water turnover varied with physical characteristics, lifestyle and environmental conditions rather than following one fixed number.

Sweating during exercise or hot weather increases water loss. Fever, vomiting and diarrhoea can do the same. Pregnancy and breastfeeding also affect fluid needs, while some medications and medical conditions can alter how the body handles water.

This is why hydration is better approached as a responsive habit rather than a daily challenge to complete a predetermined number of glasses.

Can You Rely on Thirst?

For most healthy adults living in moderate conditions, thirst is a useful guide. Drinking when thirsty and having fluids with meals will generally support normal hydration. The National Academies notes that most healthy people meet their needs through thirst and habitual drinking with meals.

Urine colour can provide another everyday clue. Pale-yellow urine generally suggests that you are drinking enough, while consistently darker urine may indicate that you need more fluid. It is only a rough guide, however, and can be affected by food, supplements and medication.

Worth Knowing: Thirst may become less reliable with age. Older adults, people with dementia and those who have difficulty obtaining drinks may need more deliberate reminders and support.

More Is Not Automatically Better

There is little benefit in forcing down large quantities of water when you are not thirsty simply to reach a target.

Although uncommon, excessive intake can overwhelm the body’s ability to maintain its sodium balance. This can contribute to hyponatremia, particularly during prolonged exercise or when medical conditions and medications affect water excretion.

Anyone who has been advised to restrict fluids because of a heart, kidney, liver or hormonal condition should follow their clinician’s guidance rather than general hydration advice.

A More Practical Hydration Routine

Keep water within reach, drink with meals and respond to thirst rather than waiting until you feel extremely dry. Carry additional water during prolonged activity or hot weather, and drink more when illness is causing fluid loss.

Water is an excellent default, particularly in place of sugary drinks, but there is no need to ignore the fluid supplied by food and other beverages.