Hydration: How Much Water Do You Really Need?

"Drink eight glasses of water a day" is one of the best-known pieces of health advice, and one of the least precise. Eight glasses of what size? Does tea count? What about the water in food?

The short answer is that eight glasses is a reasonable guide and not a rule. The longer answer is more useful, and it is below.

Why water matters

Water makes up more than half of your body weight. You need it to regulate temperature, carry nutrients around the body, cushion joints, and remove waste through urine, sweat and bowel movements.

You lose water all day, through breathing as well as sweating and going to the toilet, so it has to be replaced. When you lose more than you take in, you become dehydrated.

How much do you need?

Health bodies give slightly different figures, mainly because they measure different things.

The European and American figures look much higher than "six to eight glasses", but they include the water in food, which supplies a meaningful share of most people's intake. Fruit, vegetables, soup, yoghurt and cooked grains are all largely water.

These figures are estimates of what is adequate for a healthy population living in a temperate climate. None is a personal prescription. The US National Academies noted that most healthy people meet their needs by letting thirst guide them.

So, in practice: for most healthy adults, a drink with each meal and whenever you are thirsty will get you to roughly the right place.

The simplest test

The NHS offers a check that needs no measuring. Most people should aim to drink enough during the day so that their urine is a clear, pale yellow.

Dark yellow, strong-smelling urine, or going to the toilet much less often than usual, suggests you need to drink more. Some medicines, vitamin supplements and foods can change the colour of urine, so treat this as a guide.

When you need more

Your needs go up when you lose more fluid than usual. According to the NHS, you may need to drink more if you are:

• in a hot environment;
• physically active for long periods;
• ill or recovering from illness, especially with a fever, vomiting or diarrhoea;
• pregnant or breastfeeding.

Babies, children and older adults are more at risk of dehydration. Older people may not feel thirsty even when they need fluid, so a routine helps: a drink with every meal and one between meals.

What counts as a drink?

More than you might expect.

• Water is the best everyday choice. It has no calories and no sugar, and it is cheap.
• Milk, particularly lower-fat milk, counts and supplies protein and calcium as well.
• Tea and coffee count towards your fluid intake. Caffeine affects people differently, so the NHS suggests drinking it in moderation. If you are pregnant, the advice is to have no more than 200 milligrams of caffeine a day.
• Fruit juice and smoothies count, but they are high in free sugars. The NHS advises limiting them to a combined total of 150 millilitres a day, with a meal.
• Sugary drinks hydrate you, but they come with a lot of sugar, which is bad for teeth and adds calories. Sugar-free versions are a better choice.
• Alcohol does not help. It makes you pass more urine.

Signs of dehydration

The NHS lists these symptoms of dehydration in adults and children:

• feeling thirsty;
• dark yellow, strong-smelling urine;
• passing urine less often than usual;
• feeling dizzy or light-headed;
• feeling tired;
• a dry mouth, lips and tongue;
• sunken eyes.

Mild dehydration is usually put right by drinking. If you have been sick or have diarrhoea, start with small sips and build up. A pharmacist can advise on oral rehydration solutions, which replace the salts and sugar you have lost as well as the water.

Can you drink too much?

Yes, although it is uncommon. Healthy kidneys can get rid of a good deal of excess water, but only at a certain rate. Drinking a very large amount in a short time can dilute the sodium in your blood to a dangerous level. The US National Academies noted that this has happened when people rapidly drank far more than their kidneys could clear.

This is mainly a risk in endurance events and in drinking contests, not in everyday life. The practical lesson is that more is not always better. Drink to thirst and a little beyond; do not force down litres because a chart told you to.

Easy ways to drink enough

• Have a glass of water with every meal.
• Keep a bottle or glass where you can see it: on your desk, in your bag, by the kettle.
• Drink before, during and after exercise, and more in hot weather.
• If you dislike plain water, try it sparkling, or with a slice of lemon, lime or cucumber.
• Eat plenty of fruit and vegetables. They count towards your fluid as well as your five a day.

The short answer

Aim for roughly six to eight drinks a day, more when it is hot or you are active. Let thirst and the colour of your urine guide you. Choose water most of the time. And do not worry about hitting an exact number: your body is better at this than any app.

For how drinks fit into the wider picture, see Understanding balanced nutrition.