volume of your tank, use this simple conversion equation: 1 cu. ft. water = 7.48 gallons of water A 1,000-gallon cistern, then, equals 133.69 cubic feet of volume, which is about the size of a compact car. A 2,000-gallon cistern is approximately the size of a minivan, and the volume of 5,000-gallon tank is the equivalent of a large van or small