“It depends on your size and weight, and also on your activity level and where you live. In general, you should try to drink between half an ounce and an ounce of water for each pound you weigh, every day. For example, if you weigh 150 pounds, that would be 75 to 150 ounces of water a day. If you’re living in a hot climate and exercising a lot, you’d be on the higher end of that range; if you’re in a cooler climate and mostly sedentary, you’d need less.” – WebMD
To make it simple, we use 1 oz of water / lb of body weight per day. And consumed through out the day including all night long. Yes, all night — more on this later.