How much water are you supposed to drink a day, debunking the 8 cups-a-day myth

How much water are you supposed to drink a day, debunking the 8 cups-a-day myth
Athletes may have a harder time staying hydrated, which can negatively affect performance.The Good Brigade/Getty Images
  • How much water you are supposed to drink a day varies on your age, activity level, and where you live, so you may need more, or less, than 8 cups a day.
  • A better way to think about hydration, according to the National Academy of Sciences, Engineering, and Medicine (NASEM), is to get enough fluids in the day and not just glasses of water.
  • The NASEM recommends men consume 125 ounces of fluids per day and women get around 91 fluid ounces.

We use water to carry out nearly all of our bodily functions. So, when you don't drink enough of it, you become dehydrated, which may not sound so bad but it means that your body literally doesn't have enough water to work properly.

Eight cups of water a day was the go-to advice for decades, but the reality is a bit more complicated. This article takes a look at what you need to do to stay hydrated and why it's so important for your health.

How much water you should drink a day

Drinking enough water is important because you are losing water throughout the day through your breath, sweat, and urine.

Some fitness gurus have offered a simple equation you can use to calculate how many glasses of water you need. Divide your weight (in pounds) in half and that number is the amount of water you need per day in ounces. So, if you weigh 150 pounds, that means you should be drinking 75 fluid ounces, a little over 9 eight-ounce glasses of water a day.

The official recommendation from The National Academies of Sciences, Engineering, and Medicine (NASEM), is to think about the amount of fluids you need per day - not just the glasses of water. On average, according to NASEM, men should consume about 125 ounces of fluids (fl. oz.) per day, while women need around 91 fl. oz. These fluids can come from water, other drinks like tea and coffee, or from food.


As a general rule, most people get 80% of their fluids from beverages and 20% from food. For an idea of what that may look like, here's how you could get around 100 fluid ounces from your diet with the following routine:

  • Morning: Drink 2 eight-ounce glasses of water and 2 eight-ounce cups of coffee for a total of 32 fluid ounces. Add to that some water-rich foods for breakfast like blackberries, which are 88% water, with yogurt, which is also 88% water.
  • Afternoon: Drink 2 eight-ounce glasses of water and 2 eight-ounce cups of tea for another 32 fluid ounces. Also include water-rich foods with lunch like romaine lettuce (95% water), broth-based soups (92% water), and cucumbers (95% water).
  • Evening: With dinner, drink 2 eight-ounce glasses of water and a 16 oz. bottle of kombucha and that should get you to, or close to, 100 fluid ounces for the day.

Of course, there are other factors that will affect how well you stay hydrated throughout the day. For example, if you consume large amounts of salt, sugar, or alcoholic beverages, this could dehydrate you - even if you've consumed the necessary fl. oz. for the day. Or if you live in a dry or hot climate, you may also need more fluids than someone who lives in a cool, wet climate.

People who have a harder time staying hydrated

For the average, healthy adult, drinking when you are thirsty is a good way to stay hydrated. However, this method may not always be the best practice, says Brenda M. Davy, a registered dietician nutritionist and professor at Virginia Tech.

Athletes, for example, may need to get ahead of their thirst, as even mild dehydration, which comes on before you feel thirsty, can affect physical performance by lowering endurance and motivation. Three other groups who may have a harder time staying hydrated are the elderly, pregnant women, and new moms.

"As we age, our sensitivity to detect thirst declines, which makes older adults more susceptible to dehydration," Davy says. In this case, people over 65 are recommended to keep water nearby and drink even if they don't feel thirsty.


When you are pregnant, you also need to drink extra fluids to support both you and the baby. Being dehydrated while pregnant can lead to complications like birth defects or premature delivery. And if you are breastfeeding, you should drink an extra glass of water every time you breastfeed to replenish lost fluids.

Why hydration is important

Keeping your body hydrated provides many health benefits, including:

  • Keeping your body temperature in balance. Not getting enough fluids can make you feel overheated and sick
  • Better athletic performance. Athletes can easily become dehydrated and being fully hydrated can help boost endurance and energy.
  • Boosts in concentration and mood. Studies show that drinking enough water helps cognitive function and can make you feel calmer
  • Help with weight loss. Drinking water before meals can make you feel more full and help you cut calories.

How to tell if you're dehydrated

Besides making you feel awful, dehydration can have some serious health consequences. Over time, it can increase your risk of kidney stones, urinary tract infections, and constipation, which is why it's important to know how to tell when you're dehydrated.

The symptoms of dehydration are different for children and adults. You can check for a few key signs that you or your child may be dehydrated.

For babies and young children, parents should look out for dry mouth and tongue, as well as crying with no tears. If a baby goes 3 or more hours without a wet diaper, this can also be a sign of dehydration.


In addition to feeling thirsty, dehydrated adults may feel fatigued, dizzy, or confused. But the easiest way to know if you're getting dehydrated is to check the color of your urine, says Davy. "If you notice that your urine is not pale yellow in color, you may need to drink some water."

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