Guidelines define how much water an individual needs to be healthy, though some people lead lifestyles or have conditions that require more water than usual; therefore, these guidelines may not apply to everyone. For most people, however, the body is a well-oiled machine that will let you know when you need more water, and it's not necessary to overanalyze how much water to drink.
Know the general guidelines for you as an individual. The Institute of Medicine lays out the approximate amount of water traditionally thought necessary for males and females of varying age groups and lifestyles in liters per day. The IOM recommends males 19 to over 70 years of age consume 3.7 liters of water daily. Women in the same age group only require 2.7 liters. Adolescents and young children typically require less water. Pregnant women were told to consume 3 liters of water a day, while lactating women were said to require the most water of all at 3.8 liters daily. On the other hand, a 2011 study in the "British Medical Journal" noted that the recommendation to drink the equivalent of six to eight glasses a day has not been confirmed by studies. The author said that the recommendation is excessive, and that thirst is the best indicator of need.
Drink more water than usual if you are working out, hiking, or spending time at higher altitudes or in hot temperatures, as these activities can more easily lead to dehydration. You should also drink more water, and fluids in general, when you're sick, because vomiting, diarrhea and fever cause you to lose more water than you do when you are in good health.
Drink water whenever you experience thirst or a dry mouth because these are usually signs that your body is already dehydrated. Lack of saliva, inability to produce tears and infrequent urination are other signs you should likely be drinking more water. Fatigue, lethargy, muscle weakness, dizziness and an inability to focus can also be signs of dehydration. If your urine is not clear, you're not getting enough water.