A gallon of water is equivalent to 128 fluid ounces. The number of water bottles that make up a gallon depends on the size of the bottles. For example, a standard 16.9 oz water bottle would mean that approximately 7.5 bottles make up a gallon (128 oz / 16.9 oz per bottle ≈ 7.5 bottles). If you are using 20 oz bottles, then 6.4 bottles would make a gallon (128 oz / 20 oz per bottle = 6.4 bottles).