Do I really need to drink Eight Glasses of Water a Day?

How much water should you really drink?

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The Myth of the "8 x 8" Rule

Today's article will look into the myth of the "8 glasses of water a day" rule. A well known recommendation to drink eight glasses of water every day to keep adequately hydrated.

Is it actually based on science and facts or is it fake health information?

Drink plenty of water, heed your thirst

The short answer: you should let your thirst guide you in your drinking needs: you will cover your water requirements. And in case you are curious, the National Academy of Medicine (NAM) suggests that your daily water intake should be even higher! 2.8 quarts (2.7 litres) for women and 3.9 quarts (3.7 litres) for men ( 1 ).

But this value includes the water that you intake with your solid food and all the beverages that you drink.

Let's look into the details of this eight-glasses-of-water-per-day rule (also known as the "8x8" rule because it implies drinking eight glasses of water, each with a volume of 8 oz.)

Origin of the eight-glasses-of-water-a-day rule

Some sources state that it all started with a guidline suggested by the Food and Nutrition Board (which nowadays is part of the National Academy of Science's Institute of Medicine) back in 1945. However the Recommended Dietry Allowances (RDA) defined at that time ( 2 ) were that "A suitable allowance of water for adults is usually 2.5 litres daily. Most of this quantity is contained in prepared foods. At work or in hot weather requirements may reach 5 to 13 litres daily.".

So, the 1945 RDA was 2.5 litres = ten and a half 8-oz. glasses, so it can't be the origin of this myth.

Another article ( 3 ) erroneously states that the 1945 RDA suggested that a person drink 1ml of water per each calore of food consumed. This is roughly one-quarer teaspoon of water per Calorie. A normal adult consuming 2000 kcal a day would require 2 litres of water (2.11 quarts).

Actually, the National Research Council suggested the 1ml of water per Calorie intake for adults in the 1960s ( 4 ).

How much are eight 8-ounce glasses?

Eight 8-oz glasses = 2 quarts = 1 ⁄ 2 gallon (US) = 1.9 liters (roughly).

Valtin and Gorman (2002) ( 5 ) have done an extensive review of scientific publications and found that Dr. Fredrick J. Stare's book "Nutrition for Good Health", coauthored with Dr. Margaret McWilliams in 1974, was the first to suggest a "glass-figure" for water intake:

"How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around 6 to 8 glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc. Fruits and vegetables are also good sources of water."( 6 )

So how much water do you really need?

Don't worry about the 8 x 8 rule, you are probably very well hydrated.

Your daily water intake is covered by all common beverages such as water, tea, coffee, milk, soda or juice. The food that you eat, has between 5 and 90% water content and this also contributes towards your daily hydration goals.

Water Requirements

Some countries like the US or Germany provide Adequate Intake values (AI) for water.

The daily requirements of water are extremely variable because water needs are based on metabolism, age, , gender, environmental conditions (heat, cold, humidity), activities (light exercise, vigorous workouts, etc.). So the authorities can't provide a single level of water intake that would cover the adequate hydration needs for everyone under all types of environmental conditions.

That is why the US provides an Adequate Intake (AI) ( 7 ).

The current recommendations of the National Academy of Medicine (NAM) are (adults age +19):

  • 91 ounces of total water for women (2.7 liters)
  • 125 ounces of total water for men (3.7 liters)

"Total water" means water from all the beverages and foods consumed each day.

Water contained in food

Below we list some examples of water content in different kinds of food: (7)

  • 80 - 89% Fruit juice, yogurt, oranges, grapes, apples, pears, cooked broccoli, pineapple
  • 70 - 79% Avocados, baked potatoe, bananas, cottage cheese, ricotta cheese, shrimp, cooked corn
  • 60 - 69% Pasta, beans, chickpeas, peas, salmon, ice cream, chicken breast
  • 50 - 59% Ground beef, hot dogs, feta cheese, cooked tenderloin steak

So a dish with a half-pound hamburger, a large baked potato and an apple equals 15 fluid ounces of water.

cucumbers, tomatos and splashing water
Food, especially fruits and vegetables contain plenty of water.

The Institute of Medicine (2004) (1) determined that roughly 20% of the average American's total water intake comes from food and the other 80% from drinking water and other beverages.

European AI Values

The European Food Safety Authority suggests the following AI values ( 8 ): (which only apply to conditions of moderate environmental temperature and moderate physical activity levels - adolescents of 14 years and older are considered as adults):

  • 2.0 liters / day for women
  • 2.5 liters / day for men.
  • Pregnant women should add 300 ml / day, and lactating women 700 ml / day.

Concerned about Dehydration? Read More at our:

> > Dehydration webpage (Causes, symptoms, how to treat it and how to avoid it)


Negoianu and Goldfarb (2008) ( 9 ) remarked that nobody really knows where the "8 x 8" recommendation came from: "There is no single study -and therefore no single outcome- that has led to these recommendations.".

They also find no significant health benefit from drinking extra water and following the 8-glasses-of-water-a-day rule, but, they also reluctantly "concede there is also no clear evidence of lack of benefit. In fact, there is simply a lack of evidence in general. Given the central role of water not only in our bodies but also in our profession, it seems a deficit worthy of repletion." ( 9 )

So just drink liquids with your meals and follow your body's request for water, by drinking when you feel thirsty. This should cover your daily water requirements under most circumstances.

Young smiling girls drinking water
Listen to your body, drink when you fell thirsty and keep hydrated.

Cite this article:

. ©2018. Do I need eight glasses of water a day?. Patagonia Wellness, 11 Oct. 2018.

Tags: Dehydration, 8 x 8 rule, hydration, water requirements, daily water intake

Subject: Do you really need to drink eight 8-oz. glasses of water each day? The facts and science behind maintaining an adequate and healthy level of hydration.


References and Further Reading

(1) Food and Nutrition Board, Panel on Dietary Reference Intakes for Electrolytes and Water Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Feb. 11, 2004. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate

(2) Food and Nutrition Board, National Academy of Sciences, Revised 1945. Recommended Dietary Allowances. Nartional Research Council. Series no. 122 Aug. 1945 3-18

(4) C. Sherman, 1967. Water Excretion, Balance and Requirements, Modern Nutrition Vol 19 May-June 1967 18.

(5) Valtin H and Gorman S. (2002). "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? . Am J Physiol Regul Integr Comp Physiol. 2002 Nov;283(5):R993-1004

(6) Stare FJ , McWilliams M. (1974). Nutrition for Good Health, Plycon Fullerton.

(7) Barry M. Popkin, Kristen E. D'Anci, and Irwin H. Rosenberg (2010). Water, Hydration and Health. Nutr Rev. Author manuscript; available in PMC 2011 Aug 1. Nutr Rev. 2010 Aug; 68(8): 439 - 458. doi: 10.1111/j.1753-4887.2010.00304.x

(8) Carlo Agostoni et al. (2010). Scientific Opinion on Dietary Reference Values for water. EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA), 25 March 2010

(9) Dan Negoianu and Stanley Goldfarb (2008) Just Add Water. JASN June 2008, 19 (6) 1041-1043; DOI: