This depends on a number of variables. As a general guide for medium weight people is to drink about 1/3 of your body weight in ounces a day. So if you weigh 150 lbs (68 kgs), you should drink roughly 50 fluid oz (1.47 Liters) of water. If you work out, if it’s hot and you’re sweating more, if you fly, if you have a soda/coffee/tea, then you can choose to drink more. All of those things speed up the elimination of water and therefore, we need more.