The coverage rule
A cubic yard is 27 cubic feet. Spread 1 inch (1/12 ft) deep, 27 cu ft covers 27 × 12 = 324 sq ft. Double the depth and the coverage halves.
| Depth | 1 cu yd covers | A 2 cu ft bag covers | A 3 cu ft bag covers |
|---|---|---|---|
| 1 in | 324 sq ft | 24 sq ft | 36 sq ft |
| 2 in | 162 sq ft | 12 sq ft | 18 sq ft |
| 3 in | 108 sq ft | 8 sq ft | 12 sq ft |
| 4 in | 81 sq ft | 6 sq ft | 9 sq ft |
Bag sizes vary; read the volume on the bag you’re buying.
Account for mulch that’s already there
If the bed already has 1 in of old mulch and you want 3 in in total, you need to add 2 in, not 3. Measure in a few spots with a ruler pushed down to the soil.
How deep?
Sources differ by mulch type and soil:
- Iowa State University Extension: 3–4 in of wood chips or shredded bark on well-drained sites, 2–3 in on heavy soils, and keep mulch at least 6 in from tree trunks (source).
- University of Kentucky Extension: no deeper than 2–3 in, and about 2 in for finely ground mulch (source).
- UF/IFAS Extension (Alachua County): maintain 2–3 in and avoid more than 4 in (source).
All three warn against piling mulch against trunks, sometimes called volcano mulching.
Worked example
A 10 × 12 ft bed with a tree in it: leave out a 2 ft circle around the trunk (3.14 sq ft), so 116.86 sq ft remains. At 3 in, that’s 29.21 cu ft, or 1.08 cu yd; in 2 cu ft bags, 15 bags.