That's soooo wrong.  2 Cubic Yards does not equal 18 cubic feet.  There are 27 cubic feet in 1 Cubic Yard.  3ft.x 3ftx3ft = 27 cubic feet.  2 Cubic Yards would be 54 Cubic feet.  If a 50 lb bag of gravel is roughly 0.6 cubic feet then you would need 90 bags...54/0.6 = 90