How much water should you drink a day?

The water bottle on the desk has turned into a guilty conscience made of plastic. It sits there half full at three in the afternoon. The app sends a reminder. Someone at work carries a two-litre jug with time markings down the side.

Behind all of it sits a rule that nearly everyone can recite and almost nobody can trace. Its likely origin is a sentence somebody stopped reading too early.

What the authorities actually say

There are official figures. They are lower than most people expect, because they count differently from the eight-glasses rule.

The column that matters is the last one. The European and Nordic values apply to adults who are moderately active at normal temperatures. They cover all the water you take in, including what is already in the potato, the apple and the soup.

The Swedish Food Agency describes the split like this: 70 to 80 per cent of the water comes from drinks and 20 to 30 per cent from food. Do the sums and the drinks share comes to roughly 1.4 to 1.6 litres for women and 1.75 to 2.0 litres for men. That matches what people in the Nordic countries actually drink, which the same agency puts at around 1 to 2 litres a day.

The NHS arrives at the same place from the other direction. Six to eight 200 ml glasses is 1.2 to 1.6 litres. The NHS states plainly that this is just a guide.

Where the eight glasses came from

The rule is usually written 8 × 8: eight glasses of eight ounces. That is about 1.9 litres of water, on top of everything else you drink.

One likely origin is an American nutrition recommendation from 1945. It gave 2.5 litres of water a day as a suitable allowance for adults. One more sentence followed straight after: most of this quantity is contained in prepared foods. Two researchers who reviewed medical myths in the BMJ in 2007 pointed to exactly that. Drop the final sentence and the advice reads like an instruction to drink eight glasses.

In 2002 the kidney physiologist Heinz Valtin went looking for the evidence. He searched the databases, read the older literature and consulted nutritionists who specialise in thirst and fluid intake. He found no scientific studies in support of the rule. He limited his own conclusion to healthy adults in a temperate climate leading a largely sedentary life.

Twenty years later came the measurement. More on that in the next section.

What happens when you measure instead of asking

Older recommendations lean heavily on surveys in which people estimate what they drink. In 2022 a study in Science took a different route. The researchers had 5,604 people aged from 8 days to 96 years drink a small amount of labelled water, then tracked how quickly it left the body. The measure is called water turnover: all the water that passes through the body in a day.

Three things need saying about the table, or it will be misread.

• Turnover is not the same as drinking. According to the study, food and drink together account for roughly 85 per cent of turnover. About 10 per cent is water the body makes itself when it burns fuel. The rest is taken up through the air and the skin. The study could not determine how much comes from food.
• The averages are global. The sample includes athletes, hunter-gatherers, subsistence farmers and people living in tropical climates. People in wealthy countries with indoor climate control had lower turnover than people in poorer countries.
• The spread is the point. The lowest values sat around 1 to 1.5 litres a day and the high ones around 6. A handful of people were near 10.

The researchers' own conclusion was that one size does not fit all and that the eight-glasses advice is not backed by objective evidence. What tracked with turnover was body size, fat-free mass, how much energy the person burned, physical activity, temperature and humidity.

If you are past 40, there is one more detail. Turnover was highest in men aged 20 to 30 and in women aged 20 to 55. After that it was lower. So your need does not go up because you are getting older. What changes later in life is thirst, which we will come to.

Do coffee and tea count?

Yes. The NHS says that water, lower-fat milk and lower-sugar drinks all count, and names tea and coffee explicitly. Valtin reached the same conclusion in his 2002 review.

The idea that coffee dries you out has been tested head-on. In a British study from 2014, 50 men, all habitual coffee drinkers, drank four 200 ml cups of coffee a day for three days. In a separate three-day period they drank the same amount of water instead. Food, other fluids and physical activity were kept the same. Total body water did not differ between the two periods. Daily urine volume was 2,409 ml with coffee and 2,428 ml with water.

The study has two limitations. The participants were men who already drank three to six cups a day, so it says nothing certain about someone who rarely drinks coffee. The dose was moderate. And if you are already dehydrated or have a stomach bug, a different rule applies: the NHS then advises against both caffeinated drinks and alcohol.

Coffee counting as fluid does not mean you can drink it at any hour. Caffeine sends its bill at night, which the guide on sleeping better after 40 covers. Alcohol does not count. The NHS lists drinking too much alcohol among the causes of dehydration.

Thirst is enough for most people, but not for everyone

The Swedish Food Agency writes that healthy people with normal fluid regulation generally get enough by drinking when they are thirsty. So you already own a gauge. It is older and better calibrated than the app.

The gauge does have known weak spots. These groups should not rely on thirst alone:

• Older adults. In a classic experiment from 1984, seven healthy men aged 67 to 75 and seven men aged 20 to 31 went without fluids for 24 hours. Afterwards the older men were less thirsty and drank less, even though their blood was more concentrated. The study is small and concerns men around 70, not 45-year-olds. The finding still fits with the advice from health authorities to keep an eye on elderly relatives during hot weeks.
• Anyone who is ill. Fever, vomiting and diarrhoea raise losses faster than thirst can keep up.
• Anyone training hard in the heat. More on that below.
• Pregnant and breastfeeding women. EFSA allows for roughly 0.3 litres more a day during pregnancy and about 0.7 litres more while breastfeeding.
• Anyone on diuretics. Follow what your doctor has told you.

The Swedish Food Agency also notes that anyone who eats a lot of salt and fibre should think about drinking more.

Colour beats counting glasses

The NHS opens its advice without a single number: drink enough during the day that your pee is a clear pale yellow. Sweden's national health service, 1177, points the same way. Dark urine and peeing less than usual are signs that you need to drink more.

The method has been tested. In 1994 a research group compared urine colour with laboratory measures of how concentrated the urine was. Colour tracked the lab measures closely. The researchers concluded that it is good enough for everyday and field use, but not where laboratory precision is required.

Two sources of error. The first pee of the morning is often darker than the rest of the day's, because you have not had a drink for hours. Some supplements and foods colour urine regardless of how much you drink. So judge the colour in the middle of the day, and over several occasions.

Does drinking too little make you tired?

It can. The Swedish Food Agency lists headache, tiredness, dizziness and irritability as common signs of mild dehydration. Concentration can suffer too. The NHS lists feeling tired, feeling thirsty and a dry mouth.

The question is how large the deficit has to be. A 2018 meta-analysis pooled 33 studies with 413 participants in total, in which fluid loss ranged from 1 to 6 per cent of body mass. The effect on thinking was small overall. It was clear once the loss exceeded 2 per cent, mainly for attention, executive function and motor coordination. At 2 per cent or less it could not be reliably told apart from zero.

Two per cent of 80 kilos is 1.6 kilos. You rarely get there at a desk. You can get there during a long session in the heat.

The takeaway is practical. If you are tired at two o'clock and have not had a drink since breakfast, a glass of water is a cheap first test. But water is rarely the whole explanation. The afternoon dip has a cause of its own that has nothing to do with what is in your glass, which the guide on how long a nap should be explains.

When you exercise

Exercise is the most common reason an ordinary day calls for more. There is no general answer to how much more, because sweat rates differ so much from person to person.

The American College of Sports Medicine sets a target rather than a volume in its position stand: avoid losing more than 2 per cent of your body weight in fluid during the session. The same document describes how to work out your own loss.

1. Weigh yourself before the session.
2. Weigh yourself straight after, towelled dry, in the same clothes or none.
3. The difference is mostly fluid. One kilo is roughly one litre. Add whatever you drank during the session.

Do it once on a hot day and once on a cool one. After that you know roughly where you stand and need not weigh yourself again.

In practice, simple rules cover most everyday sessions. For a session of up to an hour at normal temperatures, drinking when you are thirsty is usually all you need. Top up with a drink at your next meal. What to eat around the session is covered in the guide on what to eat before and after a workout. Races and long runs beyond an hour involve different trade-offs, which the guide on running a half marathon works through.

If you run or cycle to work with a pack on a hot day, that is a training session, even if the calendar calls it a commute. Have a glass when you arrive.

Can you drink too much?

Yes. It is rare but serious. The Swedish Food Agency warns that drinking too much water in a short time can lead to water intoxication. The water content of the blood then becomes too high relative to its sodium. The condition is called hyponatraemia.

In sport, a blood sodium level below 135 millimoles per litre counts as hyponatraemia. A 2017 research review states that the most common cause is people drinking beyond what thirst tells them. Its advice is the same as in the rest of this article: drink to thirst.

The body has a limit on how fast it can get rid of water. According to the same review, healthy kidneys can excrete roughly half a litre to a litre an hour. So the danger does not lie in many glasses spread over a day. It lies in several litres in a short time.

According to the Swedish Food Agency, symptoms include headache, nausea, vomiting, confusion, muscle weakness and muscle cramps. If you get symptoms like these after drinking a lot in a short time, seek emergency care and do not drink more water.

What to do in practice

You do not need an app or a jug with time markings. What works is tying your drinking to things you already do.

1. A glass with every meal. That gives you three to four glasses without having to remember anything.
2. Count the coffee and the tea. They are part of the day's fluid.
3. Drink when you are thirsty. Do not ignore your thirst just because you are in a meeting.
4. Check the colour at lunch. Pale yellow: done. Dark yellow: one extra glass.
5. Add more for heat, exercise and illness. Those are the three situations where habit is not enough.

Here is what a perfectly ordinary day can look like without a single water bottle:

The table is a worked example, not a schedule. It shows that most people are already close without trying. A day when you drink less is data, not a failure. Your body lets you know and you have a glass.

When to seek medical help

• Call 999 or go to A&E if someone with signs of dehydration is confused, sleepier than normal or difficult to wake, has difficulty breathing or is taking lots of quick breaths, or has cold skin or blue, grey, pale or blotchy skin, lips or tongue.
• Ask for an urgent GP appointment or contact NHS 111 if you are breathing quickly or have a fast heart rate, or if other signs of dehydration persist even though you are drinking: feeling dizzy when you stand up, unusual tiredness, dark yellow pee or peeing less than normal. These can be signs of serious dehydration.
• Seek emergency care for headache, nausea, vomiting or confusion after drinking a lot of water in a short time. Do not drink more.
• See your GP if you are unusually thirsty, peeing a lot and tired over a longer period. These can be signs of diabetes.
• Follow your doctor's advice if you have heart failure or kidney disease, or take diuretics. The amounts that apply to you may differ from the ones given here.

This is general health information, not a diagnosis or personal medical advice. If you are unsure, contact NHS 111 or your GP. Readers in Sweden can call 1177 for round-the-clock assessment.

Frequently asked questions

How much water should you drink a day?

Around 1.5 to 2 litres of fluid a day is enough for most healthy adults in a temperate climate. All drinks count, including coffee, tea and milk. The European reference values set 2.0 litres of water a day for women and 2.5 litres for men, but that covers food and drink combined. You need more when you sweat, when it is hot and when you are ill.

Do you really need eight glasses of water a day?

No. A 2002 review found no scientific studies supporting the rule, and a 2022 measurement of 5,604 people concluded that it is not backed by objective evidence. One likely origin is a 1945 recommendation that gave 2.5 litres a day but also said most of it is already in food. Eight glasses of water is not harmful, but it is not a requirement.

Do coffee and tea count as fluid?

Yes. The NHS explicitly includes tea and coffee in your daily fluid. In a study of 50 habitual coffee drinkers, four cups of coffee a day gave the same fluid balance as the same amount of water. Alcohol does not count.

How do I know if I am drinking enough?

Look at the colour. Pale yellow pee in the middle of the day means you are on track. Dark yellow with a strong smell, or peeing less than usual, means you need to drink more. The first pee of the morning is often darker and tells you less.

Can you drink too much water?

Yes, but it is rare. Several litres in a short time can dilute the sodium in your blood, which is called hyponatraemia or water intoxication. Symptoms include headache, nausea, vomiting and confusion. Seek emergency care if it happens. Many glasses spread over a day are not dangerous for a healthy person.

Do you need to drink more water after 40?

No, not because of age. In the large 2022 measurement, water turnover was highest in men aged 20 to 30 and in women aged 20 to 55. After that it was lower. What changes later in life is thirst, which becomes a less reliable alarm. The study that showed this was in men around 70.

Sources: the Swedish Food Agency's information on water, with adequate intakes from the Nordic Nutrition Recommendations and EFSA; the NHS pages Water, drinks and hydration and Dehydration; 1177, the Swedish national health service, on dehydration; Valtin, American Journal of Physiology 2002 (review of the eight-glasses rule); Vreeman & Carroll, BMJ 2007 (the 1945 recommendation); Yamada et al., Science 2022 (water turnover in 5,604 people); Killer et al., PLOS One 2014 (coffee versus water in 50 men); Phillips et al., New England Journal of Medicine 1984 (thirst in older men); Armstrong et al., International Journal of Sport Nutrition 1994 (urine colour as a measure); Wittbrodt & Millard-Stafford, Medicine & Science in Sports & Exercise 2018 (33 studies on dehydration and cognition); American College of Sports Medicine, position stand on exercise and fluid replacement 2007; Hew-Butler et al., Frontiers in Medicine 2017 (exercise-associated hyponatraemia).