A square foot is a unit of area, and a mile is a unit of distance. A mile has 5,280 feet, and a square mile has 27,878,400 feet.
To figure out how big a square is, you multiply the length of one side by itself. The area of a square with one mile-long sides is one square mile. Each side of a square mile is 5,280 feet long. When you multiply 5,280 by itself, you get 27,878,400.
To figure out how many feet are in a cubic mile, you multiply the length of one side of the cube by itself twice. The number that comes out is 147,197,952 cubic feet per cubic mile.