You can put this solution on YOUR website! we'll convert everything to inches.
the volume of your tank is equal to 72 * 18 * 22 = 28512 cubic inches.
the number of cubic inches in a gallon is equal to 231.
divide 28512 cubic inches by 231 cubic inches to get 123.4285714 gallons.
this assumes the tank is holding liquid.
there is such a thing as a dry gallon, but it is not commonly used.
a reference that discusses imperial gallon versus united states wet gallon versus united states dry gallon is shown below: http://en.wikipedia.org/wiki/Gallon
i would assume liquid gallon unless otherwise specified, which is unlikely.
