How Much Water Should You Drink in a Day?

Breaking the Eight-Glass Myth

Medically reviewed in November 2022

How can you make sure you're staying hydrated? Should you rely on thirst alone, or is it best to abide by the eight-glasses-a-day rule?

The truth is, either approach will probably work just fine—for the most part. But your fluid needs can vary, depending on your activity level, the weather, environmental conditions and even the medications you may be taking.

Getting to know which situations are likely to increase your need for fluids can help you stay hydrated. But don't go overboard. There's no point in drinking more water than you need, just for the sake of it. At the very least, you'll end up running to the bathroom every 10 minutes. At worst, you could put yourself at risk of a life-threatening condition called hyponatremia, or water intoxication. So, how much water should you drink in a day? Read on to find out!

Too much of a good thing
Under normal circumstances, a healthy body can process large amounts of water as long as it also has plenty of electrolytes, in particular, sodium. But the combination of too much water and not enough sodium can cause problems. Those at highest risk of developing hyponatremia are:

  • Endurance athletes who lose large amounts of sodium through sweating and then flood their bodies with too much fluid as they try to rehydrate
  • People with kidney problems
  • People over 65 years of age who take multiple medications or have health conditions that compromise the body's ability to get rid of fluids or maintain adequate sodium levels
  • Although rare, hyponatremia can also occur as a result of unsafe crash dieting or binge beer-drinking.

So what about those eight glasses?
Contrary to popular belief, there is no evidence to suggest that staying hydrated hinges on drinking eight 8-ounce glasses of water each day. And relying on the eight-glass rule may not be enough to avoid dehydration in certain circumstances.

Avoiding dehydration
Mild dehydration, if it occurs frequently, may increase your risk for health problems such as heartburn, constipation, kidney stones and even kidney failure. Severe dehydration can cause your body to shut down and go into shock. So make sure you know the early warning signs of dehydration in order to take action and protect your health.

How dehydration happens
Mild dehydration occurs when you lose 3 to 5 percent of your body weight through loss of fluids (usually sweat, vomit or diarrhea).

Heat and exertion
If you're already low on fluids, and you exercise or do physical labor in a hot environment, you can become dehydrated in a matter of hours.

Fortunately, dehydration is unlikely to happen without triggering significant thirst, so if you feel thirsty, listen to your body and take frequent sips of water. Don't gulp down gallons, especially if you're suffering from nausea or diarrhea—it's likely to make things worse.

Signs of mild dehydration
The following signs are tip-offs that you may be in the early stages of dehydration:

  • Dry mouth and lips; mouth may also feel a bit gummy or sticky
  • Difficulty concentrating
  • Fatigue
  • Elevated heart rate
  • Headache
  • Dizziness

Don't ignore these clues, especially if you experience two or more at the same time.

Check your fluids
If your urine is unusually strong smelling or dark, it may be a sign that you are mildly dehydrated. But many things can affect your urine, from medications and multivitamins to certain foods or beverages, such as asparagus or beer. So also keep an eye on any changes in how much water you pass. Low urine volume over the course of a day is usually associated with dehydration.

Preventing and treating dehydration
The best treatment for dehydration is prevention. If you know you're going to be exercising or laboring in a hot, dry environment, it's a good idea to make sure you're well hydrated to start with. Take water with you, and watch out for early signs of dehydration.

To recover from and treat mild dehydration, slowly drink water or pure fruit juice until you no longer feel thirsty. A banana or a serving or two of another fruit will help restore the minerals you may have lost. Many fruits and vegetables have a lot of water in them, so you don't need to rely on drinks alone for staying hydrated.

What about sports drinks?
Sports drinks are fine if you prefer the taste, but they don't typically hydrate the body at a faster rate than water. Also, some research indicates that sports drinks may cause more gastrointestinal discomfort than water does, and they contain sugar, which will add to your daily calorie intake.

Don't be fooled by rules
Although there is probably no danger in following the 8 x 8 fluid rule, When it comes to how much water you should drink in a day, it’s important to know that your own individual fluid needs may vary. Drink fluids in comfortable amounts, and pay special attention to your fluid intake during warm weather and times of physical exertion or illness. When you work hard or work out, also remember to monitor your own signs and symptoms of dehydration. Paying attention to your body can help you promptly determine whether you need to up your intake of water or supplement your diet with more foods with high water content.

Quick Q&As

Water for weight loss?
Q. I've heard that drinking more water will help me lose weight. Is that true?
A. Well . . . yes and no. Drinking water before or with your meal may help you feel fuller faster. And if you're replacing your regular sugary soda with a glass of zero-calorie water, then yes, it may help you reach your weight loss goals quicker. But simply upping your water intake—without cutting back on calories—isn't going to wash away those extra pounds.

Age-related fluid needs
Q. Do people over 65 have special water needs?
A. For people over age 65 who have medical conditions, it is important to work with a healthcare provider on proper fluid intake, because too much water may be as dangerous as too little.

Foods with high water content
You don't have to get all of your liquids from beverages.

Over 90 percent water

  • Watermelon
  • Strawberries
  • Tomatoes
  • Celery
  • Spinach
  • Cucumbers
  • Broccoli

80 to 90 percent water

  • Carrots
  • Oranges
  • Apples
  • Peas
  • Grapes
  • Cantaloupe
  • Potatoes

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