You’ve probably heard that you need to drink eight 8-ounce (240-ml) glasses of water each day. That’s half a gallon of water (about 2 liters).

This claim has become widely accepted as fact, and it’s very easy to remember. But is there truth to this advice, or is it just a myth?

This article reviews the evidence behind the “eight glasses a day” rule and how much water we need each day.

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It’s hard to pinpoint exactly where and when the “eight glasses per day” rule originated. There are theories that it may be based on a fluid intake of 1 ml per calorie of food consumed.

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

However, a growing body of research suggests that this broad recommendation may actually be too much water for some people and not enough for others.

While there are certainly circumstances in which water needs increase, healthy people 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 (1).

But in order to stay hydrated and avoid mild dehydration, you don’t need to rigorously follow the eight glasses rule — simply follow your thirst.

Summary

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

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 (2, 3, 4).

Many 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.

For example, watermelon is 91% water, and eggs are 76% water (5, 6).

Lastly, small amounts of water are produced within your body when you metabolize nutrients. This is referred to as metabolic water (7).

People who don’t get much water from foods need to drink more than those who eat more water-rich foods.

Summary

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 drink 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 (8).
  • 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 (9).
  • Reduced severity of headaches. For those prone to headaches, drinking additional water may reduce the intensity and duration of episodes. In dehydrated people, water may help relieve headache symptoms (10, 11).
  • Constipation relief and prevention. In people who are dehydrated, drinking enough water may help prevent and relieve constipation. However, more research on this possible effect is needed (12).
  • Decreased risk of kidney stones. Although more research is needed, there is some evidence that increasing water consumption may help prevent recurrence of kidney stones in people with a tendency to form them (13, 14).
Summary

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.

However, the National Institute of Medicine has set an Adequate Intake (AI) level for total water and total beverages. The AI refers to a level that is assumed to meet the needs of most people.

The AI for total water (including water from food, beverages, and metabolism) and total beverages (including water and all other drinks) is (15):

Total waterTotal beverages
Males, ages 19–70125 ounces (3,700 ml)101 ounces (3,000 ml)
Females, ages 19–7091 ounces (2,700 ml)74 ounces (2,200 ml)

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 (16).

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

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 older adults, for example, may need to consciously remind themselves to drink water, because aging can reduce the sensation of thirst (17).

Although eight glasses of water per day is commonly touted as a science-based fluid recommendation, there’s actually little evidence to support this claim.

Water needs are highly individualized, and you can get fluids from water, other beverages, and foods, as well as from nutrient metabolism.

As a general rule, drinking to quench your thirst is a good way to ensure that your fluid needs are being met.