Calculator
Tell us your sex and whether you're pregnant, breastfeeding or exercising, and this calculator gives a typical day's water from the European Food Safety Authority (EFSA) or US guidelines, split into what you drink and what comes from food, in litres, glasses, pints and ounces. These are typical amounts for healthy adults, not targets: thirst, and pee that's a pale yellow, are the everyday check, and drinking far more than you need can be dangerous.
Choose your sex and the European or US guideline, add your age if you like, and say whether you're pregnant or breastfeeding. Add today's exercise, or your weight before and after a session, and tick a box if a doctor has limited your fluids, you're on dialysis or you've had kidney stones.
1.6litres
About 8 glasses of 200 ml, 2.8 UK pints or 54 US fl oz. That's 2.0 litres of water a day in all, counting the water in food: EFSA's adequate intake for women, with about 80% from drinks, the share EFSA assumes as the British Nutrition Foundation reports it.
Use thirst, and pee that's a clear pale yellow most of the day, as your everyday check. This figure is a typical amount for healthy adults, not a target: your own need can be half a litre or more either side. The NHS suggests 6 to 8 cups or glasses of fluid a day as a guide, and tea and coffee count.
In hot weather you need more. Let thirst guide you, check your pee is a pale yellow (a rough guide, as some foods, medicines and vitamins change the colour), and weigh yourself before and after a hot session. In a heatwave, the NHS says drink extra fluids but avoid alcohol, caffeine and hot drinks.
Don't force fluids: more than about a litre an hour is more than your kidneys can clear, and drinking far more than you sweat in a long event can dangerously lower your blood sodium. Confusion, a severe headache, repeated vomiting or a seizure after a long event needs emergency help: call 999.
The NHS lists feeling thirsty, a headache and feeling light-headed, dark yellow and strong-smelling pee, peeing less often than usual, feeling dizzy or tired, a dry mouth, lips and tongue, and sunken eyes. Ask for an urgent GP appointment or get help from NHS 111 if you're unusually tired, feel dizzy when you stand up and it doesn't go away, are peeing less than normal, or are breathing quickly or have a fast heart rate. Call 999 or go to A&E if signs of dehydration come with blue, grey, pale or blotchy skin, lips or tongue, cold skin, difficulty breathing, confusion, or being sleepier than normal: these can be signs of shock.
This calculator is for well days. If you're being sick or have diarrhoea, you need to put back salts and sugar as well as water: a pharmacist can recommend an oral rehydration solution.
If you take a water tablet (diuretic), lithium or a diabetes medicine such as dapagliflozin, ask your pharmacist how much to drink when you're unwell or it's hot.
EFSA's adequate intakes are 2.0 litres of water a day for women and 2.5 litres for men, counting the water in food. EFSA assumes about 80% comes from drinks, as the British Nutrition Foundation (BNF) reports it, so about 1.6 and 2 litres from drinks, the figures the British Dietetic Association (BDA) gives.
The NHS suggests 6 to 8 cups or glasses of fluid a day as "just a guide", more if you're pregnant or breastfeeding, in a hot environment, active for long periods or ill, and tea and coffee count. The BNF puts 6 to 8 glasses at about 1.2 litres.
The US report behind its values says the adequate intake "should not be interpreted as a specific requirement". In 5,604 people, water turnover rose with body size, activity, heat, humidity and altitude, and the study found the common 8 glasses a day "not backed by objective evidence".
Each kilo lost in a session is about a litre of sweat. Sports dietitians' drinking plans usually come to 0.4 to 0.8 litres an hour, and afterwards you drink 1.25 to 1.5 litres for each kilo lost. Drink to thirst, and don't finish heavier than you started.
Your kidneys can clear at most about 0.7 to 1 litre an hour. Drinking far more than you sweat is the main cause of low blood sodium in endurance events: in one Boston Marathon, 13% of the runners tested had it.
For most people, thirst and drinks with meals keep them hydrated day to day. After 65 neither thirst nor pee colour is reliable, so drink regularly through the day.
Total water from food and drinks: 2.0 litres a day for women and 2.5 litres for men, which EFSA says apply "only to conditions of moderate environmental temperature and moderate physical activity levels". EFSA adds 300 ml a day in pregnancy (2.3 litres in all) and about 700 ml when breastfeeding (2.7 litres).
Drinks are 80% of the total, the share EFSA assumes as the BNF reports it: 1.6 litres for women, 2.0 for men, 1.84 in pregnancy and 2.16 when breastfeeding (our arithmetic). The BDA adds the whole pregnancy or breastfeeding increase to drinks instead, which comes out 0.04 to 0.14 litres higher (our arithmetic); the calculator keeps one rule for everyone (our choice).
US adequate intakes: 2.7 litres a day for women and 3.7 for men in all, of which 2.2 and 3.0 litres come from drinks, about 81% in US surveys; 3.0 litres in pregnancy (2.3 from drinks) and 3.8 when breastfeeding (3.1 from drinks). They were set from median intakes in a US national survey, which is partly why they're higher than EFSA's (our reading).
Litres to 0.1, glasses of 200 ml to the nearest whole glass (the BDA's medium glass), UK pints of 568 ml to 0.1, and US fluid ounces to the nearest whole ounce (our choice of rounding).
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Add 0.4 to 0.8 litres for each hour of exercise, from the 2016 sports nutrition position: "The fluid plan that suits most athletes and athletic events will typically achieve an intake of 0.4 to 0.8 L/h", though it needs tailoring to each athlete. The calculator shows it as a range and calls it a typical starting plan to adjust to thirst (our choice), because sweat rates vary so much.
Sweat (litres) = weight before (kg) - weight after (kg) + what you drank (litres), because 1 kg lost is about 1 litre of sweat; your sweat rate is that divided by the hours. Afterwards, drink 1.25 to 1.5 litres for each kg still lost, the position's "~ 125 to 150% of the remaining fluid deficit". If you went to the toilet during the session, the figures run a little high (our note).
EFSA's values are for moderate temperatures, and US reference values say daily needs "can double or even triple in very hot weather", at about 40°C. But there's no sound way to turn a temperature into an amount for one person: the only equation we found predicts water turnover, not what you should drink (our reading). So the calculator gives advice instead: drink more, guided by thirst; check your pee is a pale yellow; weigh yourself around hot sessions; and in a heatwave follow the NHS advice.
A fluid limit from a doctor, or dialysis: no figure, and the result says to follow your team's limit. Kidney stones: the National Institute for Health and Care Excellence's (NICE's) 2.5 to 3 litres of water a day replaces the result. Under 18: EFSA's values for children instead. Every result says that more than about a litre an hour is more than your kidneys can clear.
Rounded as the calculator rounds them.
EFSA set 2.0 litres (women) and 2.5 litres (men) from observed intakes and desirable urine concentration, with 95th percentiles of 3.1 and 4.0 litres, and the same values for older adults. The US set 2.7 and 3.7 litres from median intakes in a national survey, and says "a wide range of intakes is compatible with normal hydration".
In US survey data about 81% of water came from drinks and 19% from food, and EFSA assumes 80% and 20%. But the water-turnover study notes surveys suggesting "20-50% of daily water intake is from food", and that "no studies to date have adequately addressed this issue". So the drinks figure suits a typical diet; lots of fruit, vegetables and soup supply more.
The NHS: "aim to drink 6 to 8 cups or glasses of fluid a day. This is just a guide". Water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count, with fruit juice and smoothies kept to one small glass (150 ml) a day. The BNF puts 6 to 8 glasses at about 1.2 litres, and the BDA counts a medium glass as about 200 ml and a mug as about 250 ml.
Sweat rates run from 0.3 to 2.4 litres an hour, depending on intensity, duration, fitness, heat acclimatisation and altitude. The 2016 position aims to keep the fluid deficit under 2% of body weight, puts typical drinking plans at 0.4 to 0.8 litres an hour, and suggests weighing before and after exercise to learn your own sweat losses.
Exercise-associated hyponatraemia is a blood sodium below 135 mmol/L during or up to 24 hours after exercise. Experts agree that drinking too much is the main cause and advise drinking to thirst; gaining weight during an event means you've drunk more than you lost. Of 488 Boston Marathon runners tested, 13% had low sodium and 0.6% critically low, linked with weight gain and finishing in over 4 hours.
The US report puts the kidney's "maximal excretion rate" at "approximately 0.7 to 1.0 L/hour"; EFSA gives "maximum urine volumes of about one litre/hour in adults" and says more than the kidneys can clear can lead to water intoxication.
Daily needs can double or triple at about 40°C, and active people in constant heat "can often have daily water requirements of 6 to 8 L/day or more". In 72 people measured in two seasons, water turnover was 3.7 litres a day in summer (about 29°C) against 3.0 in spring (18°C). How much more any one person needs is far less certain: the study's equation explains 47% of the differences between people.
Urine colour tracked urine concentration and changes in body water in small studies, but the US report says there's "no precise relationship" between urine colour and hydration, and diet, medicines and vitamins change the colour. In people aged 65 and over, urine colour, thirst and a dry mouth "are not useful, and should not be relied on individually" to spot dehydration.
In 50 male coffee drinkers, 4 cups of coffee a day hydrated as well as the same amount of water over 3 days, and in 72 men, tea, coffee, cola and lager made no difference to fluid balance over 4 hours compared with water. The NHS counts tea and coffee.
A 2002 review found "No scientific studies ... in support of 8 x 8", and the 5,604-person study concluded that "one size does not fit all for drinking water guidelines".
5 to 10 ml per kg of body weight in the 2 to 4 hours before exercise, enough to make your pee pale yellow: 350 to 700 ml for someone of 70 kg (our arithmetic).
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Aim to keep the fluid deficit under 2% of your body weight, without gaining weight. Drinking plans typically come to 0.4 to 0.8 litres an hour, but the hyponatraemia experts advise drinking to thirst, because sweat losses vary too much for fixed amounts to suit everyone.
Drink about 1.25 to 1.5 litres for every kg you're still down, over the next few hours, as the 2016 position advises: the extra covers the sweat and urine you go on losing afterwards.
Source 7
"Older athletes may present with age-related decreases in thirst sensation and may need encouragement to drink."
Source 7
In the Boston Marathon study, low sodium was more common in runners who gained weight, took over 4 hours or drank at every mile. Drink to thirst rather than at every drink station, and weigh yourself before and after long training sessions. Our triathlon and running guides cover race day.
"Over-drinking fluids in excess of sweat and urinary losses is the primary cause of hyponatremia" (the 2016 position). In long events the body also holds on to water, so drinking beyond thirst dilutes the sodium in your blood.
"Women generally have a smaller body size and lower sweat rates than males and appear to be at greater risk of over-drinking." Slower finishers, who have more time to drink, were also at higher risk in Boston.
Symptoms include bloating, puffiness, weight gain, nausea, vomiting, headache, confusion, seizures and loss of consciousness. Confusion, a severe headache, repeated vomiting or a seizure after a long event needs emergency help: call 999.
Source 7
Water toxicity has been reported after drinking large amounts quickly, far beyond the kidneys' 0.7 to 1 litre an hour. That's why the calculator never adds more than 0.8 litres an hour for exercise, and why every result says not to force fluids.
NICE: "Do not routinely advise people with heart failure to restrict their sodium or fluid consumption", but restrict fluid for people with low sodium from fluid build-up (dilutional hyponatraemia). In 504 people with chronic heart failure, a 1.5 litre limit made no real difference to health after 3 months compared with drinking freely, and caused more thirst. If you have a limit, the British Heart Foundation says your doctor or nurse will tell you how much to drink, and to check with your team in hot weather.
On haemodialysis, too much fluid can build up in your body, "leading to serious problems such as heart failure", and your care team tells you how much to drink each day.
Source 16
NICE advises adults who've had kidney stones "to drink 2.5 to 3 litres of water per day", with fresh lemon juice, no fizzy drinks and no more than 6 g of salt a day. In one trial of 220 people, drinking enough to pass at least 2 litres of urine a day roughly halved stones coming back (low-certainty evidence).
In 140 premenopausal women who drank under 1.5 litres a day and had at least 3 bladder infections a year, adding 1.5 litres of water a day cut infections from 3.2 to 1.7 a year. A bottled-water company funded the trial.
Source 26
In 631 adults with stage 3 chronic kidney disease, coaching to drink more water didn't slow kidney decline over a year, though the trial may have been too small to tell. Ask your kidney team before changing how much you drink.
Source 29
Water tablets (diuretics) make dehydration more likely; on dapagliflozin, being sick, diarrhoea or a fever bring "a risk you'll get dehydrated"; and on lithium, dehydration "can affect the levels of lithium in your blood", so call 111 if you're not eating and drinking much. Ask your pharmacist how much to drink when you're unwell or it's hot.
No personal figure. EFSA's adequate intakes for children, counting food and drinks: 1.3 litres a day at 2 to 3 years, 1.6 litres at 4 to 8, and 2.1 litres (boys) or 1.9 litres (girls) at 9 to 13; from 14, the adult values. The NHS says water and milk are the best drinks for children.
Tick the box and the calculator gives no figure: keep to the amount your team gave you, including on hot days, and ask them before changing it.
The figures still apply, but thirst and pee colour don't work as signs. The BDA suggests at least 1,600 ml of drinks a day for women and 2,000 ml for men, and a bit more in hot weather, and European geriatric guidance says all older people should be considered at risk of low-intake dehydration.
This calculator is for well days. The NHS says you need to put back the sugar, salts and minerals you've lost, and a pharmacist can recommend an oral rehydration solution.
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The calculator adds EFSA's 300 ml or 700 ml. The NHS suggests having a drink beside you when you settle down to breastfeed, and no more than 200 mg of caffeine a day in pregnancy.
No study supports a fixed 8 glasses, needs vary with body size, activity and climate, the official values differ by sex and include food, and the NHS calls its 6 to 8 glasses just a guide.
They count. Moderate coffee hydrated as well as water in a trial of 50 men, and the 2016 position calls caffeine warnings "overstated when it is habitually consumed in moderate (e.g., < 180 mg) amounts".
The NHS aim is "a clear pale yellow colour", not colourless, and drinking beyond thirst has no proven benefit and raises the risk of low blood sodium in long exercise.
The BNF says thirst "kicks in when the body is already a little dehydrated", while the 2016 position calls thirst "usually a good indication of the need to drink but not that the athlete is dehydrated". After 65, thirst is less reliable.
The kidneys clear at most about 0.7 to 1 litre an hour, and 13% of Boston Marathon runners tested had low blood sodium, linked with gaining weight in the race.
In 631 people with chronic kidney disease, coaching to drink more didn't slow kidney decline over a year. Kidney stones are the exception, where NICE advises 2.5 to 3 litres of water a day.
In two small 12-week trials alongside a diet or weight advice, 500 ml of water before meals added about 1.3 to 2 kg of weight loss; in one, the difference was no longer clear after adjusting for age, sex, ethnicity and deprivation.
At about 1.4% dehydration, 25 young women reported worse mood, harder tasks, poorer concentration and headaches, but "Most aspects of cognitive performance were not affected".
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Water is a good swap for sugary drinks, and a glass before meals may help a little alongside a diet. If you're drinking water to stave off hunger, or you're worried about your eating, talk to a GP or to Beat (our suggestion).
If you have, or think you might have, an eating disorder, these numbers aren't the right tool. See a GP as soon as you can, or talk in confidence to the eating disorder charity Beat: 0808 801 0677 (England), 0808 801 0432 (Scotland), 0808 801 0433 (Wales) or 0808 801 0434 (Northern Ireland). Beat's helplines are open 3pm to 8pm, Monday to Friday, and it also offers web chat and email. If you or someone else is in immediate danger, call 999 or Samaritans on 116 123; for medical advice, contact your GP or call 111.
Drink to thirst on these, and on a long or hot session weigh yourself before and after to learn your sweat rate.
A steady 30 to 45 minute cardio session at talk-but-not-sing pace: walk, cycle, row or cross-train, counting toward weekly activity guidelines.
40 minBeginner3 movesCardio machine, A wallA 30-minute beginner run-walk session: a brisk walk, then 1-minute jogs with 1-minute walks, building to 3-minute runs over your first three weeks.
30 minBeginner5 movesCardio machine, A wallA 40-minute easy run at a pace where you can talk in full sentences, with a brisk-walk warm-up and an easy cool-down: the base of every running plan.
40 minBeginner6 movesCardio machine, A wallA 25-minute standing cardio circuit with no jumping: marching, step jacks, sit-to-stands, punches, step-ups and wall push-ups, with a seated option for most moves.
25 minBeginner9 movesA bench or box, A stepA 10-minute routine for straight after a meal: an easy walk, a few simple standing moves from blood sugar trials, then a short walk to finish.
10 minBeginner6 movesCardio machine, A bench or boxEuropean guidelines suggest about 2 litres of water a day in all for women and 2.5 litres for men, counting the water in food; about 80% of that, 1.6 and 2 litres, comes from drinks. The NHS suggests 6 to 8 cups or glasses as a guide. You need more when it's hot, when you exercise, and when you're pregnant or breastfeeding.
Yes. The NHS says water, lower-fat milk and lower-sugar drinks, including tea and coffee, all count, and in a trial 4 cups of coffee a day kept habitual coffee drinkers as well hydrated as water. Have no more than 200 mg of caffeine a day if you're pregnant, and the NHS suggests avoiding caffeine when you're dehydrated or in a heatwave.
Drink to thirst, and on long or hot sessions weigh yourself before and after: each kilo lost is about a litre of sweat. Sports dietitians' drinking plans usually come to 0.4 to 0.8 litres an hour, but sweat rates range from 0.3 to 2.4 litres an hour, and you shouldn't finish heavier than you started.
More, guided by thirst: US reference values say daily needs can double or even triple in very hot weather if you're active. There's no reliable way to turn a temperature into an exact amount for one person, so the calculator doesn't try: check your pee stays a pale yellow, and weigh yourself before and after hot exercise. In a heatwave the NHS advises extra fluids but not alcohol, caffeine or hot drinks.
European guidelines add 300 ml a day in pregnancy and about 700 ml when breastfeeding, to 2.3 and 2.7 litres in all, of which about 1.8 and 2.2 litres come from drinks. The NHS suggests having a drink beside you when you settle down to breastfeed.
Your kidneys can clear at most about 0.7 to 1 litre an hour, so drinking much faster than that, or far more than you sweat in a long event, can lower your blood sodium dangerously. In one marathon, 13% of the runners tested had low sodium, linked with gaining weight during the race. Confusion, a severe headache, repeated vomiting or a seizure after a long event needs emergency help: call 999.
The NHS says to drink enough that your pee is a clear pale yellow; dark yellow, strong-smelling pee is a sign of dehydration. Colour is only a rough guide: some foods, medicines and vitamins change it, and after 65 it isn't reliable.
No study backs a fixed 8 glasses for everyone, and a study of 5,604 people found that needs vary with body size, activity, climate and age. The NHS's 6 to 8 cups or glasses, which include tea and coffee, are a sensible guide, and food supplies about a fifth of your water.
A little, perhaps: in two small 12-week trials, drinking 500 ml of water before meals added about 1.3 to 2 kg of weight loss to a diet. Water is also a good swap for sugary drinks. If you use water to stave off hunger or are worried about your eating, talk to a GP or call Beat on 0808 801 0677.
The European and US guidelines don't use body weight, so this calculator doesn't either. Bigger and more active people do turn over more water, about 0.7 litres a day more for each extra 50 kg in a 5,604-person study, but weight explained only part of the differences between people. For exercise, weighing yourself before and after a session tells you more than any formula.
Estimates for healthy adults, not a personal prescription or a diagnosis, and clinical review of these pages is pending. Check with a GP before hard training or testing a maximum if you have a heart condition, high blood pressure, chest pain, dizziness or breathlessness when active, and stop for sharp or increasing pain. Health disclaimer.
