How much water should you drink a day

Nurbol Zhaksybek

warehouse operator

Published: 6 September 2026 Updated: 25 September 2026

Your intake depends on your weight, not on a number of glasses

There is no single figure that suits everyone. The body spends fluid on breathing, regulating its temperature, kidney function and digestion, and the bigger the body and the busier the day, the more it spends. That is why the first reference point comes from your weight rather than from the habit of counting glasses.

A working figure and two caveats

The working figure that doctors and dietitians rely on is about 30 millilitres per kilogram of body weight a day. At 60 kilograms that comes to about 1.8 litres, at 80 to around 2.4. This turns the question of how much water you need to drink a day from an argument into a short sum you can do in your head.

The first caveat: this amount covers all fluid, not just water from a glass. Tea, soup, kefir, cucumbers and watermelon all top up the balance, so what is left for plain drinking is noticeably less than the calculated figure. The second caveat: the formula gives a range, not a point. A glass more or less either way makes no difference, and there is no sense in chasing every millilitre.

Where the figure comes from

The figure of thirty is not plucked out of thin air: it is the averaged total of daily losses. Most of it is passed by the kidneys, a few hundred millilitres go with breathing, about as much evaporates from the skin without you noticing, and the rest is lost through digestion. Add up the losses of an average adult and you get those same two litres and a little more.

From there, circumstances move the figure. There are not many of them, and they are predictable.

• Weight and body composition are the base multiplier, and the calculation starts with them
• Air temperature and humidity: losses grow in the heat and in dry rooms
• Physical activity: an hour of vigorous movement takes a noticeable share of the daily amount
• What you eat: a diet of soups and vegetables needs less drinking, dry food needs more
• Age and health: children, older people and pregnant women have their own adjustments

Everything else in this article unpacks these five lines. Let us start with the most persistent misconception, the one that makes the calculation need explaining in the first place.

Where eight glasses came from and why it is not the norm

Where the round number came from

The eight-glasses rule survives because it is easy to remember. It came from old popular guidelines that were talking about all fluid over a day, including what comes with food. That clarification got lost along the way, while the round number stayed and spread through magazines and leaflets.

Weak points of the rule

Even if you count eight glasses honestly, there is still the question of how big the glass is. One holds 150 millilitres, another 350; over eight servings the spread comes to more than a litre, which is more than half an adult's daily need. A measure that varies twofold is not a measure.

The rule's second weakness is that it is the same for everyone. Exactly how much water to drink a day depends on your weight and on how much you lost that day, and eight glasses know nothing about either. For a slim person in cool weather that amount is more than enough; for a big man after a shift in the sun it is not.

What is worth keeping

The rule does have something useful in it. It teaches you to drink regularly rather than remembering water once a day, and it sets the right order of magnitude: we are talking about litres, not a couple of mugs of tea. As a habit, eight glasses can stay; as a norm, it is better replaced with a calculation.

Why the rule lives on is simple: it cannot be checked and it cannot be failed. Nobody counts millilitres, nobody steps on the scales, and any figure seems equally plausible. The good thing about calculating from weight is that it gives you a number to hold on to, and a week later you can tell whether it helped or not.

The formula: 30 millilitres per kilogram of body weight

It takes a single step: multiply your weight in kilograms by 30, and the result is your daily fluid amount in millilitres. At 75 kilograms that is 2,250 millilitres, or 2.25 litres. All you need for the answer is a set of scales and the ability to multiply by thirty.

The calculation in four steps

1. Weigh yourself in the morning on an empty stomach: your evening weight is a litre or a litre and a half higher and inflates the result
2. Multiply your weight by 30 to get your daily fluid need in millilitres
3. Take off about a third: that much comes with food and does not need separate drinking
4. Spread the rest over the day in equal portions and watch how you feel for a week

The split between drinking and food is not fixed. Someone who eats soups, porridge, vegetables and fruit gets about a third of the amount or more from food. Someone who lives on sandwiches and dry breakfast cereal gets noticeably less, and has to make up the difference by drinking. Out of the same 2.25 litres, the first person has about a litre and a half left for water, the second closer to two.

The formula is rough, and that is fine. It does not distinguish between muscle and fat, even though muscle tissue holds more water and needs more fluid at the same weight. It knows nothing about your kidneys, your medication or your diet. For a home reference point this accuracy is enough, and where more is needed, how you feel answers better than a calculator.

Ready-made values by weight

To avoid working it out every time, keep the ready-made values to hand. The table has both figures: the total daily fluid amount and the part of it that is sensible to drink as water.

If you are looking for how to work out how much water to drink a day, you usually do not need to go beyond this table: take the row closest to your weight and treat it as your starting point. Everything else is adjustments, and they rarely come to more than half a litre.

If your weight is far from the values in the table

With a lot of excess weight the formula starts to overestimate: fat tissue contains less water than muscle, and multiplying by your whole weight gives an amount that is hard to drink and pointless. In that case, calculate from a weight close to normal for your height and make up the difference by how you feel. The opposite is true for people with a lot of muscle: they usually need a little more than calculated.

Adjustments for heat, exercise and dry air

Almaty has a sharply continental climate: summer days easily go above thirty degrees, and the air in the foothills is dry, especially when the wind blows down from the mountains. In winter the picture changes, but not for the better: central heating dries out flats and offices more than the frost outside. Both increase the unnoticed loss of moisture through the skin and breathing.

Where the extra losses come from

Exercise adds the most. An hour of vigorous training can take up to a litre of sweat, and replacing that loss in one go after the session is the wrong approach: the body has already had time to work dehydrated. It is wiser to drink a glass before you start and take a couple of sips every fifteen to twenty minutes during the session itself.

Air conditioning and the heating season deserve a line of their own. Dry air makes the mucous membranes give up moisture constantly, yet you hardly feel thirsty: your throat gets dry rather than you wanting a drink. If you wake up with a dry mouth, it usually has more to do with the air in the bedroom than with last night's dinner.

So the amount of water you need to drink a day depends not only on your weight but also on the day itself. On a hot Saturday with a walk around town your need is higher than on a rainy Tuesday at a desk, and the difference between them easily reaches a litre.

How much to add to the calculation

• Heat above thirty degrees: add 500–700 millilitres to the calculation
• Training or physical work: 200–300 millilitres for every hour of activity
• Heating season and air conditioning: roughly one extra glass a day
• A fever or any condition that makes you sweat: drink more often and in smaller portions
• Several cups of coffee in a row: a glass of water for each one beyond the third

There is no need to add up all the adjustments at once: they partly overlap. If the day was both hot and included training, go by the larger of the two additions and watch how you feel rather than the total.

Days when you are ill

Illness with a fever and any digestive upset are a separate case. Losses on such days grow quickly and unnoticed, while appetite and the urge to drink are usually absent. The small-portions rule works here: a few sips every fifteen minutes are easier to keep down than a whole glass, and are absorbed better.

What counts as water: tea, coffee, soups, vegetables

Your daily amount includes all fluid: water, tea, coffee, stewed fruit drinks, milk, soup and the moisture contained in vegetables and fruit. There is no need to force two litres of plain water on top of a lunch with soup: that would be too much, not the norm.

Tea and coffee in the balance

There is a persistent misunderstanding about tea and coffee. Caffeine does have a mild diuretic effect, but in moderate amounts it does not remove more fluid than the cup itself brings in. Two or three cups a day can safely be counted towards your balance; the trouble starts when people drink coffee instead of water, six times a shift.

In practice, the question of how much water you should drink a day runs into another one: what counts as water at all. The answer is almost everything except alcohol and sugary soft drinks. The first increases fluid loss, the second brings calories and thirst instead of quenching it, and neither is worth counting towards the norm.

How much fluid food provides

Food provides more than you might think. A bowl of soup, a serving of vegetables and a couple of pieces of fruit together easily cover a third of your daily need, and in watermelon and melon season that share grows even more. The reverse is also true: a diet of dry food and salty snacks increases the need to drink.

Your need is also affected by what sits next to the water on your plate. Salty and protein-rich food requires more fluid: the body dilutes the salt and passes the breakdown products of protein through the kidneys. After shashlik, pickles or cheese you feel thirstier not because of the spices but for this reason.

The practical takeaway is simple: cover most of your drinking with plain water, and treat tea, coffee and fruit drinks as a pleasant extra. Water has no sugar, no caffeine and no colourings, and it is the only drink that brings nothing along with it but itself.

Children, older people and pregnancy: how the norm differs

Children

Children need more per kilogram of body weight than adults: their bodies are smaller, and heat exchange and metabolism run faster. At the same time a child is worse at noticing thirst, simply forgetting about it while playing, so an adult has to remind them.

• From one to three years: about 1.1–1.3 litres of fluid in total a day (here and below, EFSA reference values, food included)
• From four to eight years: about 1.6 litres
• From nine to thirteen: about 1.9 litres for girls and 2.1 litres for boys
• From fourteen: as for adults, about 2.0 litres for young women and 2.5 litres for young men

Parents often ask how much water a child should drink a day, expecting an exact figure. An exact figure is less useful here than a habit: a bottle in plain sight and an offer of a drink every hour or hour and a half work better than any calculation, especially in summer and on days of active play.

Read more: Children's drinking water

Older people

Older people have the opposite problem: the sense of thirst becomes dulled. A person can go all day without wanting to drink and still be dehydrated, putting the first signs down to age and tiredness. A routine helps here: a few set moments in the day when you drink whether you feel like it or not.

Pregnancy and breastfeeding

During pregnancy blood volume increases, and the need for fluid grows with it; when breastfeeding, a noticeable amount is added on top of the usual norm. Swelling in the second half of pregnancy is not a reason to cut down on drinking on your own: in that situation the drinking routine is discussed with a doctor, not picked from articles.

Signs that you are not drinking enough

Dehydration starts quietly. Thirst is not the first signal but a fairly late one: by the time you want a drink, the body has been running short for a while. It is more useful to know the signs that appear earlier and are usually put down to tiredness.

Early signals

• Dark, strong-smelling urine and infrequent trips to the toilet
• A headache for no reason that goes away half an hour after a glass of water
• Sluggishness and a drop in concentration in the afternoon
• A dry mouth and lips, a sticky feeling
• Dry skin that, when pinched on the back of the hand, smooths out more slowly than usual
• Heaviness after meals and a tendency to constipation

Your body answers the question of how many litres of water you need to drink a day more accurately than any formula. If your urine is pale, straw-coloured, and you have no dryness or afternoon sluggishness, the amount is right and there is nothing to recalculate. If your urine is dark for the third day in a row, raise the calculation by half a litre.

Frequency and false hunger

Another easy indicator is frequency. When you drink enough, you go to the toilet roughly every three to four hours. If the intervals are noticeably longer and the amount is small each time, the cause is usually drinking, not the kidneys.

It is also worth remembering that hunger and thirst get confused: the brain tells these signals apart less well than people tend to think. If you feel like a snack between meals, try drinking a glass of water and waiting fifteen minutes. Often the urge goes away along with the false hunger.

How to restore the balance

If you notice two or three signs at once, do not try to make up for everything in one go. Add half a litre to your usual amount, spread it over the day and look at the result after two or three days: that is how long the body takes to settle into a new balance. If the signs persist while you are clearly drinking enough, the cause is not water, and it is a matter for a doctor.

Can you drink too much: where the safe limit lies

Hyponatraemia and who is at risk

An upper limit does exist, although it is hard to reach by accident. If you drink a great deal in a short time, the concentration of sodium in the blood drops; the condition is called hyponatraemia. It is rare: it happens to marathon runners who drink only water on the course without replacing salts, and to people with kidney and heart disease.

Speed and the upper limit on volume

A guideline on speed is simpler than one on volume. Healthy kidneys pass about a litre of fluid an hour, and anything that comes in faster stays in the body for a while. Hence the rule: drink a little and often, not a litre in one go after a long break.

The question of how much water a day you should drink also has a sensible ceiling: for a healthy adult it is about three to four litres a day outside of heat and heavy exertion. Anything above that adds no benefit, only extra work for the kidneys. Symptoms of overdoing it, such as nausea, headache and confusion, are a reason to seek help, not to wait it out.

Toxins and swelling: two misconceptions

A separate word about the idea of flushing toxins out with large amounts of water: the body does not work like a bucket under a tap. Elimination is the job of the liver and kidneys, and extra fluid simply leaves with urine without speeding the process up. The norm is there so that these organs can work comfortably, not for flushing.

With swelling, the picture is the opposite of what people expect. It is more often caused by too much salt than too much water: when fluid is short, the body starts holding on to it in reserve. So the answer is usually not to drink less but to cut out the extra salt and spread your drinking more evenly over the day. If the swelling is constant, it is a reason to see a doctor, not to experiment with your routine.

Daily routine: when to drink and how much it adds up to in a month

The total amount matters, but the way it is spread matters more. The body can only hold water in reserve for a few hours, so drinking evenly in small portions works better than two sittings of a litre each. A guideline is 150–200 millilitres every hour and a half to two hours.

A glass-by-glass schedule

1. A glass right after waking up: a noticeable amount of fluid is lost overnight
2. A glass twenty to thirty minutes before breakfast and before each meal after that
3. A couple of glasses in the first half of the day, between breakfast and lunch
4. A glass in the afternoon, when sluggishness usually sets in
5. A last glass an hour to an hour and a half before bed, so you do not have to get up at night

Once you know how much water to drink a day, all that is left is to spread it over the hours; otherwise by evening you find that all you drank during your shift was a mug and a half of tea. What helps is not an app with reminders but a container in sight: a bottle on the desk or a glass next to the monitor does more than any schedule on your phone.

Temperature and taste

It also matters what kind of water you enjoy drinking. Water that is too cold dulls thirst and gets sipped in tiny mouthfuls; water that is too warm does not refresh; cool water, around room temperature, goes down most easily. If plain water tastes flat, a slice of lemon, a mint leaf or a slice of cucumber helps: it adds no sugar and noticeably changes the habit.

A family's consumption over a month

It is useful to work out your monthly consumption once, too: that way the norm stops being an abstraction. At two litres a day it comes to about sixty litres a month per person, or roughly three standard nineteen-litre bottles. For a family, simply add the figures up.

The figure in the table is the summer maximum; in winter consumption is usually a quarter lower. The point of calculating it is not precision but understanding: the water norm is not a feat or a diet, just a few litres a month of difference and the habit of keeping a glass to hand.