“A suitable allowance of water for adults is 2.5 liters (equivalent to 8 glasses) daily in most instances"
US National Academy of Sciences Food and Nutrition Board RDA 1945 edition
How much should you really drink?
The answer depends on you.
The recommended amounts higher on average for men than women due to their higher (on average) fat-free mass and energy expenditure.
The measured weight is a critical factor in determining the condition of physical mass from the body mass index (BMI) based on the height of the physical body This measurement then is taking into account to calculate the sweat loss and activity level.
Infants and young children have a need for more water in proportion to their body weight as they cannot concentrate their urine as efficiently as adults and their surface area relative to their weight is more extensive, giving rise to greater water loss from the skin. Often children are under-hydrated. The elderly should take care to ensure adequate hydration, as aging diminishes the sensation of thirst as well as the ability to concentrate the urine. It is difficult, however, to accurately screen older people for dehydration by non-invasive tests
Water in the atmosphere (relative humidity) controls how hot we feel. In hot weather; the heat is more unbearable when combined with higher relative humidity. Thus, where the air temperature is 32 °C, this feels like 32 °C, 38 °C, 45 °C, and 56 °C, at 40%, 60%, 80%, and 100% relative humidity respectively. This is due to the increased difficulty that sweating has in cooling us down at high humidities due to the slower rate of cooling evaporation at high relative humidities.
The recommended intake level is 2.2 L for women (rising to 2.3 L if pregnant or 3.1 L if lactating). Also, female physiology shifts with the monthly changes in estrogen and progesterone levels and those fluctuations have an impact on our ability to hydrate.