ya take square footage and multiply by the depth you want the topsoil of in fractioin of inches to feet. For instance. If you want two inches of topsoil to cover a yard. Two divided by twelve is .17 so lets say you are doing 12000 square feet. You would multiply 12000 by .17 giving you 2040. Since there is 27 cubic feet in one cubic yard, you then divide that number by 27 leaving you with a total of 75.5. So it is safe to say if you are laying two inches of topsoil over a 12000 square foot yard it would take 75 yards of topsoil. If you have berms and such in the middle you just leave that out of the total calculation for the yard and then do an equation for the berm itself. So in general a formula for ya.
SQFT X INCHESDEEP/12=Cubic Feet /27 = Cubic Yards. Hope that helps.