A gallon has 4 quarts. Each quart has 2 pints, so a gallon also contains 8 pints. Each pint is 2 cups, so a gallon has 16 cups.
Each cup holds 8 fluid ounces. Therefore, there are 128 fluid ounces in 1 gallon. The standard advice is to drink eight 8-ounce glasses of water a day, which is equivalent to drinking half a gallon of water per day, because a gallon is equivalent to 64 ounces.
A quart is also equal to 4 cups. Using the metric system, a quart is about the same as 1 liter, though it is actually a little bit less, with 946.35 milliliters equaling 1 quart.