One area, one depth, one answer
Loose materials are all worked out the same way: the area you cover, times the depth you spread. What changes is how each is sold. Gravel and soil go by the cubic yard or by the ton, mulch by the yard or the bag, and each calculator answers in those units, rounded up to what a supplier will load.
Each one draws your areas to scale, takes several beds or a cut-out, and adds its line to your project list.
Updated . How we calculate. Found a mistake? Tell us.