You may have heard of the 8×8 rule. It states that you should drink eight 8-ounce glasses of water per day.

That’s half a gallon of water (about 2 liters).

This claim has become somewhat of an accepted wisdom and is very easy to remember. But is there truth to this advice or is it just a myth?

The Origin of the 8×8 Rule

The source of the 8×8 rule has not been confirmed (1).

One theory suggests it may have originated in 1945, when one research organization released a report stating that the average person needs to consume 1 ml of water per calorie of food they consume.

For someone eating a diet of 2,000 calories per day, this adds up to 2,000 ml (roughly 64 oz), or eight 8-oz glasses.

But the report also declared that much of this water could be obtained from foods you consume.

Another probable origin of the 8×8 rule is the work of a nutritionist named Dr. Frederick Stare. He coauthored a book published in 1974 that recommended drinking six to eight glasses of water per day.

The book also pointed out that fruits and vegetables, as well as other beverages, are high in water.

However, this part of the story seems to have been neglected when information from this book spread to the public, researchers and health organizations.

Bottom Line:

It’s unknown where the recommendation to drink eight 8-oz glasses of water per day comes from originally, but a couple of theories exist.

What Studies Say About the 8×8 Rule

One article from 2002 examined the scientific evidence behind the 8×8 rule (1).

It reviewed dozens of studies, surveys and articles, finding absolutely no scientific evidence suggesting that you need to drink eight 8-oz glasses of water per day for adequate water intake.

However, it must be noted that this finding is limited to healthy, albeit mostly sedentary adults living in a mild climate.

While there are certainly circumstances in which water needs increase, healthy men and women generally don’t need to be consuming water in such large quantities.

On the other hand, not drinking enough water can cause mild dehydration, defined as the loss of 1–2% of body weight due to fluid loss. In this state, you may experience fatigue, headache and impaired mood (2, 3).

But in order to stay hydrated and avoid mild dehydration, you don’t need to rigorously follow the 8×8 rule. Luckily, you have a built-in instinct called thirst.

For this reason, most people don’t need to worry about their water intake — thirst will tell you when you need water.

Bottom Line:

There is no scientific evidence to support the 8×8 rule. Water intake varies by individual and you should let thirst guide your intake.

You Can Stay Hydrated From More Than Water

It’s not just plain water that supplies your body with water. Other beverages, like milk and fruit juice, count as well.

Contrary to popular belief, caffeinated beverages and mild alcoholic drinks such as beer may also contribute to fluid intake, at least when they’re consumed in moderation (4, 5, 6, 7, 8).

These beverages only become significant diuretics when you consume them in large amounts. Diuretics are substances that increase fluid loss by making you pee more often.

A lot of the foods you eat also contain significant amounts of water.

How much water you get from food depends on the amount of water-rich foods you eat. Fruits and vegetables are particularly rich in water, and foods like meat, fish and eggs also have a relatively high water content.

Lastly, some amount of water is produced within your body when you metabolize nutrients. This is referred to as metabolic water.

In sedentary people, daily fluid intake from drinking water and other beverages is estimated to be around 70–80%, while foods are thought to account for about 20–30% (9, 10).

In the US, the proportion of water people get from food intake is estimated at around 20%, much lower than in some European countries.

People who get a low amount of water from foods need to drink more than those who eat more water-rich foods (11).

Bottom Line:

Besides water, other foods and beverages you ingest also contribute to your overall daily intake of fluids and help keep you hydrated. Some water is also created within your body through metabolism.

Drinking Enough Water Has Some Health Benefits

You need to be drinking enough water to stay optimally hydrated.

Generally speaking, that means replacing the water you lose through breath, sweat, urine and feces.

Drinking enough water may offer health benefits, including:

  • Weight loss: Drinking enough water may help you burn more calories, reducing appetite if consumed before a meal and lowering the risk of long-term weight gain (12, 13, 14).
  • Better physical performance: Modest dehydration may impair physical performance. Losing only 2% of your body’s water content during exercise may increase fatigue and reduce motivation (11, 15, 16).
  • Reduced severity of headaches: For those prone to headaches, drinking additional water may reduce the intensity and duration of episodes. In dehydrated individuals, water may help relieve headache symptoms (17, 18).
  • Constipation relief and prevention: In people who are dehydrated, drinking enough water may help prevent and relieve constipation (11, 19).
  • Decreased risk of kidney stones: Although more research is needed, there is some evidence that increasing water consumption may help prevent recurrence in people with a tendency to form kidney stones (20, 21).
Bottom Line:

Staying hydrated may aid in weight loss, help maximize physical performance, relieve constipation and more.

How Much Water Should You Drink Each Day?

There is no single answer to this question.

Adequate intake (AI) of water in the US is considered to be 91 ounces (2.7 liters) per day for women and 125 ounces (3.7 liters) per day for men (22).

Note that this is the total intake of water from all sources, not just pure water.

While this may certainly be used as a guideline, there are a number of factors, both inside your body and in your environment, that influence your need for water.

Body size, composition and activity level vary greatly from person to person. If you’re an athlete, live in a hot climate or are currently breastfeeding, your water requirements increase.

Taking all this into account, it’s clear that water needs are highly individual.

Drinking eight glasses of water per day may be more than enough for some people, but it may be too little for others.

If you want to keep things simple, just listen to your body and let thirst be your guide.

Drink water when you’re feeling thirsty. Stop when you’re not thirsty anymore. Make up for fluid loss by drinking more during hot weather and exercise.

However, keep in mind that this does not apply to everyone. Some elderly people, for example, may need to consciously remind themselves to drink water even if they’re not thirsty.

Read this for a more detailed overview of how much water you should drink per day.