How much water should you drink a day?
The famous eight glasses is a myth. The honest answer depends on your body, your activity and your climate.
Estimate how much water to drink each day from your weight and exercise.
You have heard it a hundred times: drink eight glasses of water a day. It is memorable, it rhymes with common sense, and it is not really based on anything. The real answer is both simpler and more personal, so here is what the evidence actually says and how to find the amount that suits you.
The "8 glasses" myth
The eight-glasses rule has no strong scientific origin. It also ignores a big part of the picture: you get roughly 20% of your water from food, especially fruit, vegetables and soups, and more from other drinks like tea, coffee and milk. So the target was never meant to be eight glasses of plain water on top of everything else.
The actual guidance
The most cited figures come from the US National Academies of Sciences, reported by the Mayo Clinic: about 3.7 litres (15.5 cups) a day for men and 2.7 litres (11.5 cups) for women. Crucially, that is total water from all drinks and food combined, not water you must pour from a glass. Once you subtract what you get from food and other beverages, the amount of actual water to drink is noticeably less than those headline numbers suggest.
How much do you personally need?
Your own target moves with a few things: your body size (bigger bodies need more), your activity (you lose water as sweat, so training days need more), the climate (hot or dry weather raises the need), and your health (illness, pregnancy and breastfeeding all increase it). A common practical starting point is around 30 to 35 ml per kilogram of body weight, adjusted up for exercise and heat. Our water intake calculator estimates a personal daily target from your weight and activity so you have a number to aim at rather than a one-size-fits-all rule.
The easiest way to check: your body tells you
You do not need to count every glass. Two simple signals do most of the work. First, thirst: for healthy adults it is a reliable guide, so drink when you are thirsty. Second, urine colour: pale straw or light yellow means you are well hydrated, while dark yellow means drink more. These two checks are more useful day to day than hitting an exact litre count.
Can you drink too much water?
Yes, though it is rare. Drinking a very large amount in a short time can dilute the sodium in your blood, a dangerous condition called hyponatremia, most often seen in endurance athletes who overdo plain water. For almost everyone in normal life this is not a concern, but it is a reminder that more is not always better, and that spreading intake through the day beats forcing large amounts at once.
The bottom line
Forget the rigid eight glasses. Aim loosely for the National Academies range, remember that food and other drinks count toward it, and let thirst and urine colour guide the fine-tuning. Drink more on hot days and when you exercise. This is general information, not medical advice, so check with a professional if you have a health condition that affects fluids.
Frequently asked questions
How much water should I drink a day?
A common guideline from the National Academies is about 3.7 litres (15.5 cups) a day for men and 2.7 litres (11.5 cups) for women, from all drinks and food combined. Since about 20% comes from food, the amount to actually drink is less than that.
Is 8 glasses of water a day real?
It is a myth with no strong scientific basis. It also ignores that roughly a fifth of your water comes from food and more from other drinks, so eight glasses of plain water on top is not the intended target.
How do I know if I am drinking enough?
Two easy signals: drink when you are thirsty, and check your urine colour. Pale straw or light yellow means well hydrated; dark yellow means drink more. These are more useful day to day than counting glasses.
Does coffee or tea count toward water intake?
Yes. Tea, coffee, milk and other drinks all contribute to your daily fluids, as does water-rich food like fruit and vegetables. The old idea that coffee dehydrates you is overstated for regular drinkers.
Can you drink too much water?
Rarely, but yes. Drinking a very large amount quickly can dilute blood sodium (hyponatremia), mostly a risk for endurance athletes. For normal daily life it is not a concern; spread your intake through the day.