Formula
Depth is inches. Twenty-seven cubic feet make a cubic yard:
cubic yards = length × width × depth / 12 / 27
Gravel and sand are weighed. Mulch is left as volume because that is how bulk loads are sold.
Assumptions
- The bed is a rectangle.
- Depth is the loose depth you want.
- No compaction factor is applied.
Worked example
A 30 by 10 foot bed at 3 inches needs 2.78 yd³ of mulch.
Input definitions
- Cubic yard
- 27 cubic feet, the usual bulk-mulch unit.
Limitations
Bags labeled in quarts or cubic feet are a different package. Convert the yard into those bags only with the volume printed on them.
Questions
Why not a weight?
Mulch density swings with the wood and the moisture. Sellers quote yards.
What does 3 inches do to the math?
Three inches is a quarter of a foot. The volume in cubic feet is then divided by 27.