How much water you should drink a day depends upon your body weight and the time you spend for exercise everyday.
In a general rule, if you are 140 pound weight, you should drink 70 ounce water every day. Plus, if you are exercising everyday, add more water to your diet.
To lose weight, you have to drink right amount of water on right time.
Drinking water before each meal will supress your hunger and you will eat less. And adding 500 ml of water to your daily diet can help you lose 10-15 pound body weight.