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Do You really need to Drink Eight Glasses of Water a Day?

How much water should you really drink?

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First published: 11.Oct.2018


Today's article will look into the myth of the "Eight eight-ounce glasses of water a day" rule.

You have heard this advice from friends or read about it in magazines or online health and wellness websites: you should drink eight glasses of water every day to keep adequately hydrated.

Is this an urban myth or is it based on science and facts?

The short answer is that you should let your thirst guide you in your drinking needs; that way you will cover your water requirements.

In case you are curious, the National Academy of Medicine (NAM) suggests that your daily water intake should be even higher than eight 8-oz. glasses! The NSM recommends 2.8 quarts (2.7 liters) for women and 3.9 quarts (3.7 liters) for men (1).

But this value also includes the water that you ingest with your solid food, which contains from 5 to 90% water, and all the beverages that you drink and not only plain water.

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.)

In this Article (Index)

crystal clear water pouring from bottle into a glass
Drink plenty of water, heed your thirst

The Myth of the "8 x 8" Rule

Where did the eight-glasses-of-water-a-day rule come from?

Some sources state that it all started with a guideline back in 1945 which was proposed by the Food and Nutrition Board (which nowadays is part of the National Academy of Science's Institute of Medicine).

However, the Recommended Dietary Allowances (RDA) defined at that time (2), stated that: "A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is one milliliter (ml) for each calorie of food. Most of this quantity is contained in prepared foods. At work or in hot weather requirements may reach 5 to 13 liters daily."

So, the 1945 RDA was 1 milliliter of water for each calorie of food ingested, and that was approximately 2.5 liters = ten and a half 8-oz. glasses, so it can't be the origin of this myth.

Based on the 1 ml per kcal rule, a normal adult consuming 2,000 kcal a day would require 2 liters of water (2.11 quarts). As a reference there are 20.6 ml per fluid ounce.

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) (3) have done an extensive review of scientific publications and found that Dr. Fredrick J. Stare's 1974 book "Nutrition for Good Health" co-authored with Dr. Margaret McWilliams, 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" (4).

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 contains between 5 and 90% water 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 water requirements are extremely variable because water needs are based on metabolism, age, gender, environmental conditions (heat, cold, humidity), activities (light exercise, or vigorous workouts).

For this reason, 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) (5).

The current recommendations of the National Academy of Medicine (NAM) are, for adults over 19 years of age:

  • 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

The food we eat is mostly water. Below we list some examples showing the water content in different types of food (5):

  • 80 - 89% Fruit juice, yogurt, oranges, grapes, apples, pears, cooked broccoli, pineapple.
  • 70 - 79% Avocados, baked potato, 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 (0.45 liters).

cucumbers, tomatoes 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 Water Intake Values

The European Food Safety Authority suggests the following AI values for water (6):

These values 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&fral;day for women
  • 2.5 liter⁄day for men.
  • Pregnant women should add 300 ml⁄day, and lactating women 700 ml⁄day.

Read More

Concerned about Dehydration? Read More at our:

> > Dehydration: Causes, symptoms, how to treat it, and how to avoid it.

What does the latest scientific evidence say?

A paper published in 2020, by Perrier, Armstrong, Bottin, et al., (7) reports the benefits of good hydration; more water increases urine flow and dilutes it. This has important consequences:

  • It may help prevent kidney stones.
  • It Reduces urinary tract infections.
  • It lowers the production of vasopressin (or AVP), the anti-diuretic hormone which has been linked to several diseases such as chronic kidney disease and metabolic disease.

The authors write: "this would imply drinking somewhat beyond physiological thirst and likely more than the often-repeated target of ‘eight glasses of water per day’."

Their recommendation for healthy adults doing normal (moderate) physical activity in a temperate climate is the following: Total water intake of 2.5 to 3.5 liters per day.

Total intake includes water in the food plus fluids.

glass full of sparkling water
Sparkling bubbly water

Closing comments

Negoianu and Goldfarb (2008) (8) 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."

So just drink fluids 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.

Don't overdo it, there is also the risk that you drink too much and suffer from

overhydration, a life threatening condition.

Take-home point

Keep hydrated but do not overdo it. Drink until your thirst is gone.

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

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. National Research Council. Series no. 122 Aug. 1945 3-18

(3) 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

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

(5) 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

(6) 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

(7) Perrier, E.T., Armstrong, L.E., Bottin, J.H. et al. (2020) Hydration for health hypothesis: a narrative review of supporting evidence. Eur J Nutr (2020).

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

About this Article

Do You really need to Drink Eight Glasses of Water a Day?, A. Whittall

©2023, 02 Sept. 2023. Update scheduled for 02 Sept. 2025.

Tags: hydration, dehydration, overhydration, water

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