How many glasses of water a day? The 8-glass rule checked

The short answer

• 2.0L

  Total water a day for women in Europe’s advice, from drinks and food. For men it is 2.5 L

• 9–13cups

  Cups of drinks a day (8 oz) in the US advice: about 9 for women and 13 for men

• 19%

  Of total water comes from food, in US surveys

• 0studies

  Found to back a strict “at least 8 glasses” rule, in a 2002 review

Key findingEight glasses of 250 ml (2 litres) is a reasonable starting point for many adults, and India’s guidelines say the same. But tea, coffee, milk and the water in food all count, and you need more in hot weather or when you sweat.

Where did 8 glasses of water a day come from?

No one has found a study behind it. In 2002, a US physiologist (a scientist who studies how the body works) searched for the source of “drink at least eight 8-oz glasses of water a day”. He also asked experts on thirst and drinking. He found no scientific studies that support it for healthy adults.

Older official advice may be part of the story. In 1989, the US guide said adults need about 1 ml of water for each calorie they burn. For someone burning 2,000 calories, that is 2 litres: eight 250 ml glasses (our calculation). But that figure counted water from all sources, food included. In 2005, the US National Academies wrote that confusion about how much to drink comes partly from misreading earlier advice, such as this 1989 guide.

A 2022 study of water use in 5,604 people agreed. Its authors wrote that the advice to drink eight 8-oz glasses a day “is not backed by objective evidence”.

How many glasses of water a day do official guides suggest?

Europe’s food safety agency (EFSA) and the US National Academies both set an “adequate intake”. That is the amount healthy people usually take in, judged to be enough. Both count all drinks and the water in food. EFSA’s figures assume moderate temperatures and moderate activity.

Food gives about a fifth of this water. In the US advice, drinks make up about 2.2 litres a day for women and 3.0 litres for men. India’s 2024 guidelines say a healthy person needs about eight glasses (about 2 litres) of water and other drinks a day.

What about US glasses? A US 8 oz glass holds about 237 ml. So 8 glasses of 8 oz is 64 oz, or about 1.9 litres (our calculation). The US advice works out at about 9 cups of drinks a day for women and 13 for men.

Your needs can be very different

In 2022, researchers measured how much water 5,604 people used each day. Scientists call this “water turnover”. It differed a lot from person to person.

Heat matters. People used about 1 litre a day more at 30 °C than at 0–10 °C. Athletes used about 1 litre more than non-athletes. In the last three months of pregnancy, water use rose by about 670 ml a day.

Official guides agree that needs rise with heat and activity. India’s guidelines say to drink more in very hot weather and during hard physical work. US estimates for people in hot climates ranged from 4 to 12 litres a day, from inactive to very active.

Do tea, coffee and food count?

Yes. The US National Academies concluded that tea, coffee and other drinks with caffeine add to your daily water much like other drinks do. Milk, buttermilk and lemon water count too. Many fruits and vegetables are 85–95% water. If you drink tea or coffee late in the day, see how long caffeine lasts.

India’s guidelines suggest buttermilk, lemon water, coconut water and fresh juice without added sugar over soft drinks, which they say to avoid. Tender coconut water has about 15 calories per 100 ml. People with kidney or heart disease who are prone to high potassium should avoid it.

Can you drink too much water?

It is rare, but yes. Healthy kidneys can get rid of about 0.7 to 1 litre an hour at most. Drinking much faster than that for hours can dilute the sodium in your blood to a dangerous level. During exercise, this is usually linked with endurance events lasting more than 6 hours. For healthy people in daily life, no upper limit has been set.

Claims and evidence

• ClaimEveryone needs exactly 8 glasses of plain water a day.

  EvidenceA 2002 review found no studies behind this. Official figures work out at about 6 to 12 glasses of drinks, depending on sex and on which guide you use. Size, activity and heat change needs too.

• ClaimTea and coffee don’t count, because they dry you out.

  EvidenceThe US National Academies found that drinks with caffeine add to your daily water much like other drinks.

• ClaimFood doesn’t count towards your water.

  EvidenceAbout a fifth of total water came from food in US surveys. Fruits and vegetables are 85–95% water.

• ClaimThe more water you drink, the healthier you are.

  EvidenceThe US experts found too little evidence to set water amounts for preventing long-term disease. Drinking far more than your kidneys can clear can be dangerous.

What this means for you

What the research can’t tell you yet

• Based on habits, not trials. Official amounts reflect what healthy people usually drink. No trial has tested how much water is best.
• Turnover isn’t need. The 2022 study measured water use, not whether people were well hydrated.
• Climate. EFSA’s figures assume moderate temperatures and activity. Hot weather raises needs.
• Long-term health. There is too little evidence to set water amounts that prevent long-term disease.

How Bodyvault uses this research

You can log water with one tap: a 250 ml glass on Apple Watch, or a Control Center button on iPhone that you can set from 50 to 1,000 ml. You can set your own daily target, from 1 to 6 litres. If you turn on nudges, Bodyvault checks at 11 a.m., 2 p.m. and 5 p.m. and reminds you only if you’re more than 500 ml behind an even pace for your day. To pick a target, try the water intake calculator.

General information about fitness and nutrition, not medical advice. If you have a health condition, are pregnant or take medication, talk to your doctor before changing your diet or training.