This is a PIA question. Different places sell it differently. Some by yards some by tons. Supposely the ton is a hair over a yard but that depends if it has been raining and it is real moist or not.
I theory topsoil will cover the exact same as mulch, but I find that it will cover slightly less. In other words for every 20 yards I know I need I add 1-2 extra yards. This will help you in getting a smooth grade at equal depth and compensate for minor divits and hills of preexisting fill dirt.
Here is a good calculator. http://upstatemulch.com/material_calculator.htm