Is it 29,920 US gallons?
I'll try to explain the difference by an example.
Go into your livingroom:
The floor, the walls and the ceiling are areas.
The air in the livingroom is the content of the volume of this room.
(If area and volume is the same and you try to glue the air to the wall as a wallpaper you'll be visited by two friendly men, clad in a white uniform, who will show you the way to ... well, don't do that )
In other words:
distance is an one-dimensional object; units inch, foot, cm, km, mile, etc
area is a two-dimensional object; units square-inch, square-foot, square-cm, ...
volume is a three-dimensional object; units cube-inch, cube-foot, cube-cm, ...