Yard Calculators
Mulch and gravel both start with the area and depth you measure. The math is the same; what changes is the material.
Mulch Calculator
Organic mulch is cited at 2 to 4 inches. Finely shredded mulch is cited at 1 to 2 inches. The bag count uses the cubic feet printed on the bag you buy.
Leave the bag field blank if you only want cubic yards.
Gravel Calculator
No. 57 stone is 1.39 tons per cubic yard, and No. 8 stone is 1.32, both from the Delaware DOT material weight chart.