“Drink eight glasses of water a day” is one of the most common health rules you’ll hear. But is eight glasses actually the right amount for everyone?

Not necessarily.

Your daily water requirement can change depending on your age, body size, activity level, climate, diet and health status. Someone spending an hour exercising outdoors in hot weather may need considerably more fluid than someone working at a desk in an air-conditioned room.

So, how much water should you drink in a day?

There isn’t one exact number that works for every person. Official guidance on water intake is generally expressed as an overall intake from beverages and food, while individual needs can vary considerably. The National Academies’ adequate intake for total water is 3.7 litres a day for men and 2.7 litres for women aged 19–30, but these figures include water from food as well as beverages and are not a fixed requirement for every individual.

Here’s what you need to know about daily hydration and the factors that can change how much water your body needs.

How Much Water Should You Drink in a Day?

A commonly cited reference from the National Academies gives an adequate intake for total water of approximately:

3.7 litres per day for adult men
2.7 litres per day for adult women

But there is an important detail: these numbers refer to total water, not simply the amount you should drink from a water bottle.

Water from beverages and moisture naturally present in foods both contribute to your total intake. In the data used by the National Academies, beverages supplied approximately 3.0 litres for men and 2.2 litres for women, with the remainder coming from food.

These figures are also adequate-intake values, not a prescription saying every healthy adult must consume exactly that amount every day. The National Academies notes that people who are physically active or exposed to hot environments may need more.

That is why a simple “drink exactly X litres every day” rule can be misleading.

What Counts Towards Your Daily Water Intake?

Your body gets water from more than plain drinking water.

Your total fluid intake can include:

Plain water
Milk and other beverages
Tea and coffee
Soups and other liquid foods
Water-rich fruits and vegetables
Other foods containing significant amounts of water

The National Academies notes that water comes from food, beverages and drinking water, while the CDC also points out that foods such as fruits and vegetables contribute to total fluid intake.

So if you eat watermelon, cucumber, oranges, soups or other water-rich foods, you’re getting some fluid from your diet too.

What Affects How Much Water You Need?

Your hydration needs aren’t fixed. Several everyday factors can increase or decrease them.

1. Hot Weather

Living in a hot climate can increase fluid losses through sweating.

This is particularly relevant in India during summer or when you’re spending extended periods outdoors.

If you’re sweating more than usual, you may need to drink more to replace the fluid you’re losing. The CDC specifically identifies hot climates as a situation in which people may need more water.

Don’t wait until you’ve spent hours in the heat to start thinking about hydration.

Carry a reusable water bottle and drink regularly throughout the day.

Product opportunity:
A good reusable water bottle is one of the easiest hydration-related products to recommend naturally. Look for useful features such as a leak-resistant lid, comfortable grip, easy cleaning and a capacity that suits your routine rather than simply choosing the largest bottle available.

2. Exercise and Physical Activity

Your water requirements can increase when you exercise because your body loses fluid through sweat.

The amount you need depends on factors such as exercise intensity, duration, temperature and how much you sweat.

Someone doing a short indoor workout will have different needs from someone running outdoors for an hour in hot, humid conditions.

The CDC recommends drinking more water than usual when exercising in hot weather and advises people not to wait until they are thirsty before drinking more.

For prolonged or particularly sweaty exercise, electrolyte replacement may sometimes be relevant. However, you don’t automatically need sports drinks for every workout.

3. Your Diet

Food contributes to your overall water intake.

Fruits and vegetables with high water content can make a meaningful contribution, while salty foods may increase thirst.

Your beverage choices also matter. Water is a calorie-free option, while replacing sugary drinks with water can reduce calorie intake.

If you struggle to drink plain water, adding lemon or lime, or eating more water-rich foods, can make hydration easier without relying on sugary drinks.

4. Fever, Vomiting or Diarrhoea

Illness can change your fluid needs considerably.

Fever can increase fluid loss, while vomiting and diarrhoea can result in significant loss of both fluids and electrolytes.

The CDC lists fever, vomiting and diarrhoea among situations where people may need more water.

If you are experiencing significant or persistent fluid loss, don’t assume that simply drinking large amounts of plain water is always the appropriate solution. Depending on the situation, medical advice or appropriate oral rehydration may be necessary.

5. Pregnancy and Breastfeeding

Fluid requirements can also change during pregnancy and breastfeeding.

Pregnancy can increase vulnerability to dehydration, particularly in hot conditions. The CDC recommends paying particular attention to hydration and heat protection during pregnancy.

Breastfeeding also changes nutritional and fluid needs.

If you’re pregnant or breastfeeding and are unsure how much fluid you personally need, your doctor or dietitian can give advice based on your individual circumstances.

6. Age

Water needs aren’t identical throughout life.

Children, teenagers and older adults have different hydration considerations from younger adults.

Older adults may also be more vulnerable to dehydration because thirst sensation and the body’s ability to regulate fluid balance can change with age.

Rather than applying an adult “8 glasses” rule to everyone, hydration should be considered in the context of age, health and lifestyle.

7. Your Health and Medications

Certain health conditions can affect how much fluid you should consume.

People with some kidney, heart or liver conditions, for example, may be advised by their healthcare professional to restrict or carefully manage fluid intake.

Some medications can also affect fluid balance.

If a doctor has given you a specific fluid restriction or hydration plan, follow that rather than a general internet recommendation.

How Do You Know If You’re Drinking Enough Water?

Thirst is one useful signal, but it isn’t the only thing you can pay attention to.

Your urine colour can provide a rough everyday clue. The CDC notes that urine that is light yellow or clear generally indicates adequate hydration.

Possible signs of dehydration can include:

Thirst
Darker urine
Urinating less frequently
Headache
Dizziness or light-headedness
Fatigue
Dry mouth

More severe dehydration can cause more serious symptoms and may require medical attention.

Remember that urine colour can be affected by factors other than hydration, including some foods, supplements and medications. It shouldn’t be treated as a diagnostic test.

Does Everyone Need Eight Glasses of Water?

No.

The “eight glasses a day” idea is easy to remember, but it isn’t a universal prescription for every adult.

Your total fluid needs depend on several variables, including your food intake, activity level, climate and individual physiology.

For some people, eight glasses may be enough or more than enough on a particular day. For others, particularly people exercising or spending time in hot conditions, it may not be enough.

The better question isn’t simply:

“Have I had eight glasses?”

It is:

“Am I replacing the fluid I’m losing based on my day’s conditions and activity?”

Can You Drink Too Much Water?

Yes.

More water isn’t automatically better.

Drinking excessive amounts of water can dilute the sodium concentration in your blood and cause hyponatremia, a potentially serious condition. Excessive fluid intake is particularly relevant during prolonged exercise, when people may drink more than they are losing through sweat.

This is one reason hydration advice should not simply be “drink as much water as possible.”

If you are exercising for long periods, especially in hot conditions, your hydration strategy should account for fluid and electrolyte losses rather than focusing only on consuming increasingly large amounts of plain water.

Should You Drink Water Before You Feel Thirsty?

It depends on the situation.

For everyday life, thirst is an important physiological signal that helps regulate fluid intake.

During prolonged exercise or significant heat exposure, however, relying entirely on thirst may not be sufficient for some people. The CDC advises people exercising in hot conditions to drink more than usual and not wait until they are thirsty.

The practical approach is to drink regularly throughout the day rather than trying to “catch up” by drinking a huge quantity at once.

The Bottom Line

So, how much water should you drink in a day?

There isn’t one perfect number for everyone.

As a broad reference, the National Academies’ adequate intake for total water is about 3.7 litres per day for adult men and 2.7 litres for adult women, including water from food and beverages.

But your personal needs can change with heat, exercise, diet, illness, pregnancy, breastfeeding, age and medical conditions.

Rather than forcing yourself to hit an arbitrary water target, make hydration a regular part of your day, pay attention to your body’s signals and increase your intake when you are losing more fluid.

And remember: hydration is about getting enough, not simply drinking as much as possible.