Updated 30 January 2017

Do we really need 8 glasses of water a day?

Taking into account that fluid requirements vary from person to person, how can we ascertain how much fluid an individual needs on a daily basis?


We’re often told we should drink eight glasses of water per day. Where does this recommendation come from, and does it apply to everyone?

How do we calculate our water requirements?

Your fluid requirements are determined by your metabolic demands and the necessity to balance sensible and insensible water loss. Your metabolic demand is influenced by your body size, body composition, amount of physical activity and the presence of conditions such as fever.

You also lose fluid through perspiration (influenced by factors such as altitude, activity, humidity and environmental temperature) and breathing.

Considering all the factors that play a role in fluid balance, it is easy to see that individual requirements vary greatly and one size really doesn’t fit all! So, how on earth are you supposed to ascertain how much fluid you as an individual need?

Read: Water beats energy drinks

The eight glass a day rule stems from the need to have an easy-to-understand message that stresses the importance of sufficient fluid intake. The body has no provision for water storage, thus the amount of water lost every 24 hours must be replaced. The US Institute of Medicine (IOM) recommends an average intake of fluid (from food and drink) of 3L per day for men and 2.2L per day for women, or alternatively approximately 35ml of water per kg.

If solid food accounts for approximately 750ml (or 3 cups) of water per day and oxidative metabolism contributing about 250ml (or 1 cup) of water per day, a minimum of 1.2–2.0L (or 5–8 cups) per day of additional fluid is needed. As discussed, this value is subject to change with higher intake needed to account for physical activity and exposure to extreme environments. Please note that these calculations are based on the “average” sized person, but if in doubt use the 35ml fluid per kg calculation for a more individualised amount.

Can you drink too much water?

Healthy people with normal kidney function are seldom at risk for water intoxication when their needs exceed their water requirements. Although thirst is a signal for the need to consume water, people can be at risk of chronic dehydration.

Dehydration causing as little as 2% loss of body weight can result in an impairment of both your physical and psychological performance. In addition to this, chronic mild dehydration has been linked to constipation, dry mouth and kidney stones (in those who are susceptible). So, the eight glasses a day rule is meant to be an easy average to help people to remember to drink more fluid, but is by no means a hard and fast rule.

Which drink is the best for hydration?

It is recommended that most of our daily fluid intake (approximately 70%) should be plain water. The water from additional hot and cold beverages can make up the remainder.

Read: Water myths debunked

Should you then consume cold drinks, juices, teas and coffee, it is vital to remember that these alternative drinks may contain additional calories (usually from added sugar), which may result in unwanted weight gain or dental caries if they replace your plain water.

So, while you may be adequately hydrated, additional calories and sugar may lead to unintended results like weight gain. If you do choose to replace some of your plain water with an alternative drink, make sure that it contains little or no added sugar for best results.

Does caffeine cause dehydration?

There is a concern that drinking caffeine containing beverages will cause dehydration. There is good evidence that this is untrue, which means that caffeinated and non-caffeinated drinks are equally hydrating.

What about sparkling water?

Unsweetened sparkling water can be counted as part of your fluid intake. Sparkling water is made by dissolving carbon dioxide into the water under pressure. By itself, carbonated water appears to have little impact on health. While carbonated water is somewhat acidic, this acidity is quickly neutralised by your saliva.

The bottom line

Drink unsweetened drinks, such as plain water, sparkling water, teas (regular or herbal) and coffees (without sugar) to thirst within the guideline of approximately 8 glasses (or 2L) per day and you will reap the benefits of being well hydrated. 

Read more:

Water and your body

Cellulite, water intake linked

Hyperglycaemics need more water


1.       Charney, P. Water, electrolytes and acid-base balance. Krause's Food & The Nutrition Care Process. St. Louis, Mo: Elsevier/Saunders, 2012.

2.       Practice-based Evidence in Nutrition: Healthy Lifestyle Key Practice Points. Dietitians of Canada 2015. Last updated on 20 June 2011.

3.       Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: The National Academies Press. 2005.

4.       Practice-based Evidence in Nutrition: Caffeine Knowledge Pathway. Dietitians of Canada 2015. Last updated on 22 October 2012.

5.       Ireland R. Advanced Dental Nursing. 2nd ed. Wiley-Blackwell; 2010.


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