How Much Water Do You Really Need a Day?

Water bottles have become a fashion accessory, and phone apps remind us to drink every hour. The message is everywhere: drink more water.

But how much do we actually need? And does it have to be plain water?

The official figures

Most of the water we need comes from drinks, but some comes from food. Fruit, vegetables, soups, rice and dal all contain a lot of water.

In 2010, the European Food Safety Authority (EFSA) set these figures for total water intake for adults living in a mild climate with moderate activity:

EFSA assumes that around 20% of our water comes from food, though this varies a lot with diet.

The US National Academies gave higher figures in 2004: about 2.7 litres a day for women and 3.7 litres for men, again including food. These are based on how much healthy people typically consume, rather than a minimum anyone must reach.

The UK’s NHS gives simpler advice: aim for 6 to 8 cups or glasses of fluid a day.

Where “eight glasses a day” came from

The famous rule is often traced to a 1945 recommendation by the US Food and Nutrition Board, which suggested about 2.5 litres of water a day. The very next sentence, often forgotten, said that most of this quantity is contained in prepared foods.

In 2002, the kidney specialist Heinz Valtin reviewed the evidence for drinking at least eight glasses of water a day. He found no scientific studies supporting it for healthy adults in a mild climate.

A large 2022 study published in Science measured how much water passed through the bodies of more than 5,000 people in 26 countries. Daily water turnover ranged widely, from about 1 litre to more than 6 litres, depending on body size, activity, climate and other factors. The researchers concluded that a single figure such as eight glasses doesn’t fit everyone.

There is no single amount of water that’s right for everyone.

What counts towards your fluids

• Water, of course.
• Tea and coffee. Caffeine has only a mild effect on urine, so these still count overall. See how caffeine works.
• Milk, buttermilk and lower-sugar drinks all count.
• Juice and fizzy drinks count, but are high in sugar. The NHS suggests keeping fruit juice and smoothies to 150 ml a day.
• Food with a lot of water, such as cucumber, watermelon, oranges, soups and curries.

Alcohol is not a good way to stay hydrated.

When you need more

You need more fluid when you lose more water:

• Hot weather. Sweating can use up several extra litres a day.
• Exercise and physical work.
• Illness, especially fever, vomiting or diarrhoea. In these cases, oral rehydration solution can replace salts as well as water. See how ORS works.
• Pregnancy and breastfeeding. EFSA adds about 300 ml a day in pregnancy and 700 ml while breastfeeding.

Older people often feel less thirsty than they used to, so they can become dehydrated without noticing. Young children also need watching in hot weather.

How to tell if you’re drinking enough

For most healthy adults, thirst is a reliable guide. Two other simple signs help:

• Urine colour. Pale straw-coloured urine usually means you’re well hydrated. Dark yellow urine suggests you need to drink more. Some vitamins and foods can change the colour.
• How often you go. Passing urine only rarely, or in small amounts, can be a sign of not drinking enough.

Signs of dehydration include thirst, dark and strong-smelling urine, dizziness, tiredness, a dry mouth and headaches.

Can you drink too much?

Yes, though it is rare. Drinking very large amounts in a short time can dilute the sodium in your blood, a condition called hyponatraemia. It can cause headaches, confusion and, in severe cases, seizures. It has mainly been seen in endurance athletes who drink far more than they sweat out, and in drinking contests.

People with kidney, heart or liver conditions may be told by their doctor to limit fluids. Always follow that advice.