Quick answer
Calculate the new layer, not the material already present
Multiply bed area by the additional depth you need. Apply a visible allowance, then divide by the exact net volume printed on the bag and round complete bags upward. For bulk delivery, divide cubic feet by 27 to report cubic yards.
Calculate my mulch nowThe formula
Calculate cubic yards and bags
length × widthNet volumebed area × additional installed depthOrder volumenet volume × (1 + allowance ÷ 100)Complete bagsround up (order volume ÷ net bag volume)Divide depth in inches by 12 when area is in square feet. Estimate by volume rather than weight because particle size, material type, and moisture can change weight without changing the package label.
Reference table
Square feet covered by one bag
Coverage equals net bag volume divided by installed depth.
| Bag volume | 2 in deep | 3 in deep | 4 in deep |
|---|---|---|---|
| 1.5 ft³ | 9 ft² | 6 ft² | 4.5 ft² |
| 2 ft³ | 12 ft² | 8 ft² | 6 ft² |
| 3 ft³ | 18 ft² | 12 ft² | 9 ft² |
Use the bag's net volume, not its weight or the bag's outside dimensions. Package sizes differ by product and market.
Horticultural input
Choose depth for the actual bed and planting plan
EPA WaterSense states that three to four inches gives suitable coverage for most plants, while too much can restrict water from reaching roots. It also recommends leaving space between organic mulch and tree or woody-plant bases to help prevent rot.
That guidance is context, not an automatic calculator setting. Existing depth, soil, drainage, plant type, mulch particle size, climate, decomposition, and local recommendations may require a different added layer.
If 2 inches remain and the target is 3 inches, estimate a 1-inch addition—not a new 3-inch layer.
Source: U.S. EPA WaterSense Landscaping Tips.
Worked example
A 20 ft × 10 ft bed with a 3-inch layer
Use 10% allowance and bags labeled 2 ft³.
- Bed area: 20 × 10 = 200 ft².
- Depth in feet: 3 ÷ 12 = 0.25 ft.
- Net volume: 200 × 0.25 = 50 ft³ = 1.852 yd³.
- Order volume: 50 × 1.10 = 55 ft³ = 2.037 yd³.
- Coverage per bag: 2 ÷ 0.25 = 8 ft².
- Complete bags: round up (55 ÷ 2) = 28 bags.
The 28 bags contain 56 ft³, so one cubic foot above the 55 ft³ requirement comes from final package rounding.
Before purchase
Verify the bed, depth, and package
- Break irregular beds into measured shapes and combine their volumes.
- Measure existing mulch before choosing the additional depth.
- Confirm the mulch type is appropriate for the plants and site.
- Use the exact bag volume or supplier bulk-delivery increment.
- Keep mulch clear of trunks and stems according to local horticultural guidance.
Common questions
Mulch quantity FAQ
How do I calculate cubic yards of mulch?
Multiply bed length by width by installed depth after converting depth to feet, then divide cubic feet by 27. Add only the project allowance you intend to use.
How many square feet does a bag of mulch cover?
Divide the bag's net cubic volume by the installed depth in feet. A 2 ft³ bag covers 8 ft² at 3 inches because 2 divided by 0.25 equals 8.
How deep should mulch be?
EPA WaterSense says three to four inches suits most plants while excessive mulch can restrict water flow. Plant, soil, drainage, climate, and mulch type still matter, so use local guidance for the final depth.
How do I calculate a mulch top-up?
Enter only the additional depth needed above the existing layer. Using the full finished depth for a top-up would count material already in the bed.
Open the BuildMeasure Mulch Calculator and enter the exact package volume.