Is it true that we should drink eight glasses of water per day?

The rule of “eight glasses” has been strongly rooted in the media, and as a common belief concerning the demand for liquids. The equivalence of eight glasses of water (which is two liters), is supposed to be the starting level of fulfilling the need for fluids for a person with regular physical activity. In other words, even if we sit behind the desk for the whole day, clicking on the keyboard, we still have to drink two liters of water not to dehydrate our bodies. However, is that rule anyhow justified?

The issue of setting the fluids demand

Setting individual demand for fluids is not easy, the needs of bodies depend on many factors, such as:

  • body mass and composition,

  • age,

  • physical effort,

  • stress load,

  • the temperature of the environment,

  • quantity and quality of diet,

  • physiological condition.

Official guidelines

According to the present state of knowledge, it is assumed that we should drink about 1ml of water per each calorie supplied with food (or burned during the day). There is also a conversion, according to which we should try to provide 30-40ml of water per each kilogram of body mass. These assumptions concern adults with average physical activity, average body mass and mixed diet with advantage of carbohydrates. In case of sports people and active people, the needs of body increase significantly. In some extreme circumstances (intensive workout done in heat), the losses of fluids may range to 2.5-3l per hour!

Eight glasses of water per day?

Remembering about the circumstances mentioned above, it's easy to conclude, that the recommendation, according to which we should drink eight glasses of water per day it too high. What's more, in some cases it may occur to be insufficient. Well, in practice, you have to take under consideration few more important aspects, in order not to base your knowledge on misunderstanding. First of all, it's good to remember about the fact, that when we talk about hydration, we should have in mind the total supply of fluids, not only water. The daily amount of eight glasses should include juice, colorful beverages (if anyone drinks them), tea, and even coffee and water hidden in food (for example, fruit contain about 70-90% of water, which also hydrates the body). Second of all, our body may function properly during fluctuation of the fluids supply, and the matter of real demand is individual. Some people of similar activity and antropometric parameters, will need three liters of liquids, and others will be satisfied with half of that amount without the risk of dehydration (clothes may matter here, as well as the environmental temperature, the level of stress, intake of salt, fiber, protein, etc.)


The guideline, according to which we should drink eight glasses of water per day, does not derive from scientific studies. For some people, drinking such amount of water may be unnecessary complication of life (I'm thinking about individuals with average activity, who consume large amount of liquids with food and other beverages), and for sports people who are professionals, such doses are definitely insufficient. Every day we should base on thirst, and it's good to create more thorough counting for the before- or after-workout period.