Mulch Calculator
Cubic yards of mulch = bed area in ft² × depth in inches ÷ 324. A 20 × 5 ft bed at 3 in needs 0.93 yd³, or 13 bags of 2 cu ft (9 bags of 3 cu ft).
Mulch needed
0.93 yd³
25 ft³ · 708 L
- 2 cu ft bags
- 13
- 3 cu ft bags
- 9
- Bed area
- 100 ft²
How it works
Volume (ft³) = bed area (ft²) × depth (in) ÷ 12
Cubic yards = ft³ ÷ 27 (one yard covers 324 ft² at 1 in)
Bags = ft³ ÷ bag size (2 or 3 ft³), rounded up
Assumptions
- The bed is measured as a rectangle; for curved beds use the square footage calculator and enter the area as a long, 1 ft wide bed.
- Volume is the loose volume printed on the bag. Fresh mulch settles a little after watering.
Worked examples
A 20 × 5 ft bed at 3 in
- 20 × 5 = 100 ft²
- 100 × 3 ÷ 12 = 25 ft³ = 0.93 yd³
- 25 ÷ 2 = 12.5, so 13 bags of 2 ft³; 25 ÷ 3 = 8.3, so 9 bags of 3 ft³
Result: 0.93 yd³: 13 bags of 2 cu ft or 9 bags of 3 cu ft.
Checked by the automated test “mulch — 20 × 5 ft bed at 3 in = 25 ft³ → 13 bags of 2 ft³ or 9 bags of 3 ft³”.
Iowa State Extension's example: 265 ft² at 1.5 in
- 265 × 1.5 ÷ 12 = 33.125 ft³
- 33.125 ÷ 27 = 1.227 yd³
Result: 1.23 cubic yards; Iowa State's page shows 1.22 (it cuts the decimals instead of rounding).
Checked by the automated test “mulch — 265 ft² at 1.5 in = 33.125 ft³ = 1.22 yd³ (Iowa State)”.
Bags or bulk
- Bags are sold by volume, not weight. The label gives cubic feet (often 2 or 3). The calculator rounds bags up to the next whole bag.
- Bulk comes by the cubic yard. For a large bed, compare the bag count with a bulk price plus delivery.
- Top-ups need less. If the old mulch is still 1 in deep and you want 3 in, enter 2 in. A 100 ft² bed then needs 16.7 ft³, or 9 bags of 2 cu ft.
Common mistakes
- Piling mulch against trunks. UMD Extension says mulch should never touch the trunk and should stay about 3 in away; it shows the mulch “volcano” as the wrong way to do it.
- Going too deep. UMD Extension notes that excess mulch piled on root systems can make plants decline. Stay within the recommended depth.
- Reading the bag count for the wrong size. The result lists 2 cu ft and 3 cu ft bags separately; use the line that matches the bags on the shelf.
Frequently asked questions
How much mulch do I need?
Multiply the bed area by the depth in feet. Iowa State University Extension does it this way: 265 ft² × 0.125 ft (1.5 in) = 33.125 ft³, then ÷ 27 for cubic yards. The calculator does the same from length, width and depth in inches.
How much does a yard of mulch cover?
324 ft² at 1 in deep and 108 ft² at 3 in, as NC State Extension notes for a cubic yard of organic material. Divide 324 by your depth in inches for any other depth.
How many bags of mulch are in a yard?
A cubic yard is 27 ft³: 13.5 bags of 2 cu ft (so buy 14) or 9 bags of 3 cu ft. Check the size printed on the bag, since some are sold in other sizes.
How deep should mulch be?
Iowa State University Extension says hardwood chipped or shredded mulch is often spread 2 to 4 in thick. The University of Maryland Extension recommends one to three inches around trees and shrubs, kept about 3 in away from trunks.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- How to Determine the Amount of Mulch Needed for a Garden Bed — Iowa State University Extension and Outreach
- Organic Lawn Care – Appendix 1 (one cubic yard covers 324 ft² at 1 in, 108 ft² at 3 in) — NC State Extension
- Mulching Trees and Shrubs — University of Maryland Extension