How to use this calculator
- Add each bed or tree. Choose rectangle, circle or ring around a tree, or type the square feet if you already know them. For irregular beds, break them into rough rectangles.
- For trees, enter the ring size and a bare circle at the trunk. The calculator leaves that circle out, because mulch should never touch the trunk.
- Pick a depth. Two to three inches is the usual recommendation.
- If you’re topping up, enter how much mulch is already there. The calculator only adds the difference.
- Optional: enter prices per bag and per cubic yard to compare bagged and bulk.
The formula
- Cubic feet = area (sq ft) × depth (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
- Ring around a tree = 3.1416 × (outer radius² − bare-center radius²)
- Bags = cubic feet ÷ bag size, rounded up
Worked example
A 20 × 4 foot front bed and a 6-foot mulch ring around a maple, with a 1-foot bare circle at the trunk, mulched 3 inches deep:
- Front bed: 20 × 4 = 80 sq ft
- Tree ring: 3.1416 × (3² − 0.5²) ≈ 27.5 sq ft
- Total: 107.5 sq ft × 3 ÷ 12 ≈ 26.9 cubic feet, just under 1 cubic yard
- In 2-cubic-foot bags: 26.9 ÷ 2 = 13.4, so 14 bags
How deep should mulch be?
A layer of 2 to 3 inches is enough to hold moisture, keep soil temperatures even and smother most weed seedlings. The University of Illinois Extension notes that fresh mulch looks deeper than it is and settles, and that adding a new layer every year without checking can build it up too deep. More than about 3 inches can starve roots of air and water.
Avoid the mulch volcano
Mulch heaped up against a tree trunk looks tidy but keeps the bark constantly damp and dark, which invites rot, insects and disease, and can encourage roots to circle the trunk. Instead, spread mulch in a wide, flat ring, as wide as practical, and leave a bare circle a few inches across around the trunk itself.
Bags or bulk?
One cubic yard is 27 cubic feet, the same as 13.5 bags of 2 cubic feet. For a yard or more, bulk mulch from a landscape supplier usually costs much less per cubic foot than bags, but check delivery fees on small loads. Bulk is normally sold in half- or whole-yard steps, so the calculator rounds bulk orders up to the next half yard.
Sources
- University of Illinois Extension: Too much of a good thing, mulch volcanoes slowly kill trees
- UConn Extension: Mulch volcanoes, a growing problem
Frequently asked questions
How much mulch do I need?
Multiply the area in square feet by the depth in inches and divide by 12 to get cubic feet. Divide by 27 for cubic yards. For example, 100 square feet at 3 inches deep needs 25 cubic feet, about 0.93 cubic yards or 13 bags of 2 cubic feet.
How deep should mulch be?
University extension guides, such as the University of Illinois Extension, recommend 2 to 3 inches for landscape beds and around trees. Deeper mulch can keep water and air from reaching roots.
How many bags of mulch are in a yard?
One cubic yard is 27 cubic feet, so it equals 13.5 bags of 2 cubic feet, 18 bags of 1.5 cubic feet, or 9 bags of 3 cubic feet.
Should mulch touch the tree trunk?
No. Piling mulch against a trunk, often called a mulch volcano, keeps the bark wet and can lead to rot and other damage. Keep a bare circle a few inches wide around the trunk and spread the mulch outward in a wide, flat ring.
Do I need to remove old mulch before adding new?
Usually not. Check how deep the old layer is first. If it is already 2 to 3 inches, just rake it to freshen it. If it has broken down to an inch or so, add only enough to bring it back to 2 to 3 inches. Enter the existing depth in the calculator to get the top-up amount.