A worked example
A 500 sq ft bed at a 3-inch depth needs about 4.63 cubic yards of mulch.
Frequently asked questions
Why is 2-3 inches the typical recommended depth?
It's deep enough to suppress weeds and retain soil moisture, but shallow enough to avoid smothering roots or trapping excess moisture against plant stems, which can encourage rot.
Where does the 324 conversion factor come from?
It converts square feet and inches of depth directly into cubic yards in one step: 1 cubic yard equals 27 cubic feet, and 12 inches make a foot, so 27 × 12 = 324 — dividing area times depth-in-inches by 324 skips a couple of unit conversion steps.