How the mulch calculator works
Mulch is a volume problem: the area of the bed multiplied by the depth of the layer. The calculator converts everything to the same unit first, so feet and inches never get mixed.
Cubic yards = area (sq ft) × depth (in) ÷ 324
The 324 comes from two conversions at once. Dividing inches by 12 gives feet, and dividing cubic feet by 27 gives cubic yards; 12 × 27 is 324. In metric it is simpler: square metres × depth in metres gives cubic metres.
For bags, the calculator divides the total volume by the bag size and rounds up, because a shop will not sell you 0.4 of a bag.
A worked example
A bed 12 feet long and 4 feet wide covers 48 square feet. At 3 inches deep:
- 48 × 3 ÷ 324 = 0.44 cubic yards
- 0.44 × 27 = 12 cubic feet
- 12 ÷ 2 = 6 bags of 2 cu ft, or 4 bags of 3 cu ft
How deep should mulch be?
For shredded or chipped hardwood in beds, Iowa State University Extension gives a range of 2 to 4 inches. Finer material can sit at the lower end; coarse bark chips hold their structure at the upper end.
Around trees and shrubs, keep it to 2 to 3 inches over the root zone and pull it back from the trunk. NC State Extension warns that mulch piled against the stem, often called volcano mulching, holds moisture against the bark and can cause rot and girdling roots.
Bags or bulk?
Bagged mulch is usually sold in 2 or 3 cubic foot bags. One cubic yard is 27 cubic feet, so a yard equals about 13.5 two-foot bags or 9 three-foot bags. Once a job passes 2 or 3 cubic yards, bulk delivery by the yard is usually less work to handle, and many suppliers set a delivery minimum.
Measuring irregular beds
- Curved or kidney-shaped beds: split them into rectangles and circles, run each one, and add the results.
- Circles around trees: measure the diameter of the ring you want, not the radius.
- Topping up: measure the depth that is left and enter only the difference you want to add.