A yard of dirt measures 3ft x 3ft x 3ft, which is 27 cubic feet. So, if you were to spread that dirt 1ft thick, it would cover 27 square ft. Since there are 12 inches in a foot, you would be able to divide that cubic foot into 4 if you only needed the soil to be 3 inches deep (3x4=12).
So if you got 4 square feet out of 1 cubic foot, and there are 27 cubic feet in a yard, you will get 4 x 27 or 108 square feet. So you could cover an area very slightly larger than 10ft x 10ft with a yard of dirt, if it were spread 3 inches thick.
