How Much Water Should You Drink a Day?

Quick answer

Rough daily fluid targets are about 9 cups (2.2 L) for women and 13 cups (3.0 L) for men, adjusted for weight, activity, and climate — the ‘eight glasses’ rule is a myth.

You have probably heard that you should drink eight glasses of water a day. It is a tidy rule, easy to remember, and mostly wrong. The honest answer to how much water you should drink a day is: it depends on your body size, activity level, climate, and even what you eat. Health authorities point to a total water intake of about 3.7 litres a day for men and 2.7 litres for women, but that figure includes the water already in your food and other drinks, not just plain water poured from a glass. This guide explains where those numbers come from, how to estimate your own needs, and why chasing an arbitrary target can occasionally backfire. To get a personalised figure in seconds, try our free water intake calculator, then read on to understand the reasoning behind the result.

How much water should you drink a day?

Most healthy adults need roughly 2.7 litres (women) to 3.7 litres (men) of total water per day, according to the U.S. National Academies. About 20 percent of that comes from food, so the amount you actually drink is closer to 2.0 to 3.0 litres. Your exact need shifts with your weight, activity, and climate.

Notice the phrase total water. That is the single most misunderstood part of hydration advice. The famous targets are not eight extra glasses on top of everything else you consume. They are the sum of every source of water reaching your body over a day.

Where the numbers come from

The 3.7 and 2.7 litre figures come from the National Academies of Sciences, Engineering, and Medicine (NASEM), which sets reference intakes for the United States and Canada. These are adequate intake values based on what typical healthy people actually consume while staying well hydrated, not strict minimums you must hit.

Two details matter. First, the numbers describe total water: plain water, tea, coffee, milk, juice, and the moisture inside foods like fruit, vegetables, soup, and yoghurt all count. Second, they are population averages. A small, sedentary person in a cool office and a large athlete training in the heat can both be perfectly healthy while drinking very different amounts.

Group	Total water per day	Roughly, as fluids
Adult men	About 3.7 litres	About 3.0 litres (13 cups)
Adult women	About 2.7 litres	About 2.2 litres (9 cups)
Pregnancy	About 3.0 litres	About 2.3 litres
Breastfeeding	About 3.8 litres	About 3.1 litres

One cup here means about 240 ml. If you prefer thinking in glasses, a standard glass is roughly the same size.

The body-weight formula

Population averages are a starting point, but a quick body-weight estimate personalises the target. A widely used rule of thumb is 30 to 35 millilitres of fluid per kilogram of body weight per day.

Daily fluid (ml) = body weight in kg x 30 to 35

Here is how to use it:

Take your body weight in kilograms. If you weigh yourself in pounds, divide by 2.205 first.
Multiply by 30 for a baseline figure, or by 35 if you are active or live somewhere warm.
The result is your target in millilitres. Divide by 1000 to convert to litres.

Worked example. Take an adult who weighs 70 kg.

Baseline: 70 x 30 = 2100 ml, which is 2.1 litres.
Active or warm climate: 70 x 35 = 2450 ml, which is about 2.45 litres.

In plain English: a 70 kg person should aim for roughly 2.1 to 2.5 litres of fluid on an ordinary day, and lean toward the higher end when sweating more. This lines up neatly with the NASEM averages above, which is why both methods are useful cross-checks.

Skip the arithmetic

You do not have to run these numbers by hand. Our water intake calculator takes your weight and activity level and returns a personalised daily target instantly. Pair it with our calorie calculator and BMR calculator to build a fuller picture of your daily needs.

The 8×8 rule: helpful myth or bad math?

The eight glasses of eight ounces rule, often written 8×8, adds up to about 1.9 litres. It is easy to remember, and for many women it lands close to the right ballpark. But it has two problems.

First, no strong scientific study established 8×8 as a requirement. Researchers who traced its origins found it likely grew from a 1945 recommendation that also said most of that water comes from food, a caveat that got dropped as the slogan spread.

Second, a fixed number ignores your size and situation. A 90 kg construction worker and a 50 kg office worker do not have the same needs, yet 8×8 hands them the identical target. Use it as a loose floor if you like, but a body-weight estimate or the calculator will serve you better.

Factors that change how much you need

Several everyday variables push your target up or down:

Exercise. You lose water through sweat. Add roughly 350 to 700 ml for every hour of moderate activity, and more in heat.
Climate. Hot or humid weather, and high altitude, both raise fluid loss.
Body size. Larger bodies hold and use more water, which is exactly why the weight-based formula works.
Diet. A diet rich in fruit, vegetables, and soup delivers more water through food. A salty, processed diet increases your need.
Health. Fever, vomiting, diarrhoea, and some medications increase water loss. Certain heart and kidney conditions require fluid limits set by a doctor.
Pregnancy and breastfeeding. Both raise requirements, as the table above shows.

How to tell if you are drinking enough

You do not need to track every millilitre. Your body offers reliable feedback. The simplest check is urine colour: pale straw yellow usually means you are well hydrated, while dark amber suggests you should drink more. Feeling thirsty, having a dry mouth, headaches, or low energy can also signal you have fallen behind.

For most healthy people, a good habit beats obsessive counting: drink when thirsty, keep water within reach, and have a glass with each meal. If you want to understand your broader daily needs, our guide on how many calories you should eat a day and our protein requirement guide cover the same personalised approach for food.

Can you drink too much water?

Yes, though it is uncommon. Drinking a very large volume in a short time can dilute the sodium in your blood, a condition called hyponatremia. In mild cases it causes nausea and headaches; in severe cases it can be dangerous. It mostly affects endurance athletes who overdrink during long events, not people sipping normally through the day.

The takeaway is balance. More water is not automatically better. Aim for your estimated target, spread intake across the day rather than forcing litres at once, and let thirst guide you between meals.

Common mistakes to avoid

Treating 8×8 as a hard rule. It ignores your body weight and activity.
Forgetting food and other drinks count. Roughly a fifth of your water comes from what you eat.
Assuming coffee dehydrates you. Caffeinated drinks in normal amounts still add to your daily total.
Ignoring thirst, or overriding it. Both extremes cause problems. Thirst is a useful signal, not a failure.
Chasing a huge number for weight loss. Water can support a healthy diet, but drinking litres beyond your needs does not melt fat.

Frequently asked questions

How much water should I drink a day in glasses?

As a rough guide, women need about 9 cups (2.2 litres) and men about 13 cups (3.0 litres) of fluid per day, where a cup is roughly 240 ml. That is total fluid, so other drinks count toward it. Your exact number depends on your weight, activity, and climate, which is why a weight-based estimate is more accurate than a fixed glass count.

Does coffee or tea count toward water intake?

Yes. In normal amounts, coffee and tea are hydrating and count toward your daily total, despite their mild diuretic effect. The water in these drinks outweighs the small amount of extra fluid loss from caffeine. Water and unsweetened drinks are still the best everyday choices, but your morning coffee is not working against your hydration.

Should I drink more water when exercising?

Yes. You lose water through sweat, so add fluid before, during, and after activity. A common guideline is an extra 350 to 700 ml for each hour of moderate exercise, and more in hot conditions. Drink to thirst during shorter sessions, and for long endurance events plan your intake so you neither underdrink nor overdrink.

How much water should I drink to lose weight?

There is no special amount that causes weight loss. Water has no calories, and drinking a glass before meals may modestly reduce appetite, but it will not burn fat on its own. Aim for your normal estimated target, use water in place of sugary drinks, and rely on overall diet and activity for results.

Is it bad to drink water with meals?

No. The idea that water dilutes digestion is a myth. Drinking water with meals is fine and can help you feel full and pace your eating. Your stomach acid and digestive enzymes handle the extra fluid without trouble for the vast majority of people.

What are the signs of dehydration?

Common signs include thirst, a dry mouth, dark yellow urine, headaches, tiredness, and dizziness. Urine colour is the easiest daily check: pale straw yellow suggests good hydration, while dark amber means you should drink more. Severe dehydration with confusion or a racing heartbeat needs prompt medical attention.

Can drinking too much water be dangerous?

It can, though it is rare. Drinking a very large volume very quickly can dilute blood sodium, a condition called hyponatremia, causing nausea, headaches, and in severe cases serious harm. It mainly affects endurance athletes who overdrink during long events. Spreading normal amounts across the day avoids the risk entirely.

The bottom line

How much water you should drink a day is not a single magic number. Most adults do well aiming for about 2.7 litres (women) to 3.7 litres (men) of total water, with roughly 2.0 to 3.0 litres of that coming from drinks. A quick body-weight estimate of 30 to 35 ml per kilogram personalises the figure, and your thirst and urine colour keep you on track day to day. Skip the rigid rules, let your body guide you, and use our water intake calculator to get a target tailored to you.

Sources

National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.
U.S. Centers for Disease Control and Prevention. Water and Healthier Drinks.
European Food Safety Authority. Scientific Opinion on Dietary Reference Values for Water.