The staff of life! How much water is enough per day? Here is a simple rule of thumb: Take ½ of your body weight in pounds (lbs), and That amount is what you should drink in ounces (oz) as an example: Ie. if your weight is : 128 lbs x 0.5= 64 ounces water =8 glasses x 8 ounces (1cup) water/day= 2 L *(1 L= 32 ounces=4 cups)