Holy crap! Here in Michigan we have dealers who give us booklets & have service people capable of giving us a mulch amounts needed for a job. Its 1 yd @ 2" covers 212 sq ft. Geesus! Its not that hard to figure out! Take the measurements with a tape measure and multiply them together. Thats it. Very simple. 2 inches is all that should be needed! It would be advised to remove the old mulch first, but isnt always necessary.
💪Cuttin In Michigan Jeff