For most people, 70-80 ounces of water a day is enough. This amount may change with activity level, temperature, medical conditions, or dietary needs.
1 ounce is 0.03 Liters. 1 gallon is 3.4 Liters, or 132 ounces.
An easy rule to remember is to have half a gallon (1.7 L), or eight 8 ounce glasses every day.
There are conditions when you should drink more water.
Exercise. 30 minutes of moderate exercise should result in the need for an additional 8-12 ounces.
Intense exercise. Prolonged, heavy exercise will result in the need for much greater amounts of water. Sometimes an additional liter or more.
Pregnancy and breast feeding. Women who are pregnant or who are breast feeding may need to drink 2 times as much water as normal, normally 2-3 liters per day (12 or more 8 ounce glasses)
Hot weather. Hot or humid weather will increase the need for water due to sweating. The amount of water needed varies.
What if that amount of water is not enough?
When enough water is not consumed, the urine tends to become darker, more concentrated or foul-smelling. The number of times the bladder must be emptied during the day will also decrease. When more than enough water has been consumed, the urine tends to be clear, colorless, and the urinary frequency increases.