I measure out how many cubic feet I will need for example, if the mulch bed is 12' by 4' and you want it to be 3" deep then LxWxH 12x4x.25=12 cubic feet.
I measure the beds with a contractor wheel. Total up all the area. I come up with a num lets say 1200sqft of shrub beds. Customer wants fresh fines mulch I open that website go to fresh fines type 600 for height 2 for length (requires both pieces of info) it comes up with 7.5 yards at 2 in. Just remember plants take up space and I usually order about .5 to 1 yard less but bid for full in case I need it