How much water should you drink each day? - a simple question with no easy answers. Studies have produced varying recommendations over the years, but in truth, your water needs depend on many factors, including your health, how active you are and where you live.
 
Though no single formula fits everyone, knowing more about your body's need for fluids will help you estimate how much water to drink each day.
 
Health benefits of water
Water is your body's principal chemical component, comprising, on average, 60 percent of your weight. Your muscles are made up of approximately 70% water, and your brain 75%! Every system in your body depends on water. For example, water flushes toxins out of vital organs, carries nutrients to your cells and provides a moist environment for ear, nose and throat tissues. Nearly all of the major systems in your body depend on water. 

How much water do you need? 
Every day you lose water through your breath, perspiration, urine and bowel movements. For your body to function properly, you must replenish its water supply by consuming beverages and foods that contain water.
 
A couple of approaches attempt to approximate water needs for the average, healthy adult living in a temperate climate.
 
The average urine output for adults is 1.5 liters a day. You lose close to an additional liter of water a day through breathing, sweating and bowel movements. This means the average adult loses 2.5 liters of water a day with NO EXERCISE.

This does not even factor in the amount of water a person loses because of the things they drink. Coffee, tea and diet sodas are diuretics. Diuretics are those things which will TAKE away from your water consumption because they cause the body to lose water.
 
Factors that influence water needs 
You may need to modify your total fluid intake depending on how active you are, the climate you live in, your health status, and if you're pregnant or breast-feeding.

Exercise: The more you exercise, the more fluid you'll need to keep your body hydrated. An extra 1 or 2 cups of water should suffice for short bouts of exercise, but intense exercise lasting more than an hour (for example, running a marathon) requires additional fluid. How much additional fluid is needed depends on how much you sweat during the exercise, but 13 to 26 ounces (or about 2 to 3 cups) an hour will generally be adequate, unless the weather is exceptionally warm.

Environment: Hot or humid weather can make you sweat and requires additional intake of fluid. Heated indoor air also can cause your skin to lose moisture during wintertime. Further, altitudes greater than 2,500 meters (8,200 feet) may trigger increased urination and more rapid breathing, which uses up more of your fluid reserves.

Illnesses or health conditions: Signs of illnesses, such as fever, vomiting and diarrhea, cause your body to lose additional fluids. In these cases you should drink more water. Certain conditions, including bladder infections or urinary tract stones, also require increased water intake. On the other hand, certain conditions such as heart failure and some types of kidney, liver and adrenal diseases may impair excretion of water and even require that you limit your fluid intake.
 
So how much should I drink? 
As you’ve learned, there are many factors that will affect the amount of water you need. However, a good rule of thumb is to drink a liter of water for every 50 pounds of body weight.
Lack of water can lead to dehydration, a condition that occurs when you don't have enough water in your body to carry out normal functions.