You can put this solution on YOUR website! I assume it is round and flat at the bottom.
Then we have a cylinder like a can of soup.
Since it is a circle we get the area the same way
d=2r
pi*r^2
pi*12^2
and to get volume we multiply by height
144*pi*15 and we get cubic inches.
6785.8401 cubic inches=29.38 gallons
convert anything to anything
http://www.easysurf.cc/cnver6.htm#cig4