How the number is worked out
rectangle area = length × width
circle area = π × (diameter ÷ 2)²
ring area = circle area (outer) − circle area (bare trunk circle)
cubic feet = area × (depth wanted − already there) ÷ 12
cubic yards = cubic feet ÷ 27
Bulk mulch is sold by the cubic yard, bagged mulch by the cubic foot, so the calculator gives both. Bags are the cubic feet divided by the size printed on your bag, rounded up.
A check on the arithmetic from Clemson Cooperative Extension: “There are 27 cubic feet in a cubic yard. One cubic yard will cover a 324-square-foot area with an inch of mulch” (HGIC 1604). That is the same formula run backwards: 27 ft³ × 12 ÷ 1 in = 324 ft².
Worked examples
A 10 ft × 4 ft bed, 3 in deep, starting bare:
- 10 × 4 = 40 ft², × 3 ÷ 12 = 10 ft³ = 0.37 yd³
- With bags marked 2 cu ft: 10 ÷ 2 = 5 bags
Five trees, each with a 6 ft ring and a 1 ft bare circle at the trunk, 3 in deep:
- Ring: π × 3² − π × 0.5² = 27.49 ft²
- Per tree: 27.49 × 3 ÷ 12 = 6.87 ft³
- Five trees: 5 × 27.49 × 3 ÷ 12 = 34.36 ft³ = 1.27 yd³
Topping up the same bed from 1 in to 3 in only needs 2 in of new mulch — two thirds of the bare-ground amount. That is what the “already there” box is for.
How big to make a tree ring
Clemson gives two sizes (HGIC 1604): “Newly planted trees require a circle of mulch 3 to 4 feet in diameter,” and “For established trees in lawns create a circle of mulch about 2 feet in diameter for each inch of trunk diameter.”
So a tree with a 6 in trunk gets a ring about 6 × 2 = 12 ft across. At 3 in deep, keeping the default 1 ft bare circle at the trunk:
- Outer circle: π × 6² = 113.10 ft²
- Bare circle: π × 0.5² = 0.79 ft²
- Ring: 113.10 − 0.79 = 112.31 ft²
- Mulch: 112.31 × 3 ÷ 12 = 28.08 ft³ = 1.04 yd³
- Bags: 28.08 ÷ 2 = 14.04, so 15 bags of 2 cu ft; 28.08 ÷ 3 = 9.36, so 10 bags of 3 cu ft
Enter 12 for the outer diameter and 1 for the bare circle in the Ring shape to get the same figures.
Why the ring has a hole in the middle
The ring shape leaves a bare circle around the trunk because mulch should be kept back from the bark, not piled against it. Clemson puts it this way: “Keep mulch 2 to 3 inches away from the stems of woody plants. This will prevent decay caused by wet mulch and rodent damage during the winter,” and, for new trees, “do not pile mulch against the trunk” (HGIC 1604).
Our own inference from that, not a figure Clemson gives: the bare circle needs to be at least the trunk diameter plus 4 to 6 in (2 to 3 in on each side). The default 1 ft circle is 6 in from the trunk’s center, which leaves 3 in of clearance on a 6 in trunk and 2 in on an 8 in trunk. For a thicker trunk, make the bare circle bigger. Set it to 0 only if you really want a solid disc.
Questions people ask
How many bags of mulch are in a yard?
A cubic yard is 27 cubic feet, so it is 27 ÷ 2 = 13.5 bags of 2 cu ft (buy 14) or 27 ÷ 3 = 9 bags of 3 cu ft. Check the size on your bag; it is printed in cubic feet.
How much does a yard of mulch cover?
It depends on the depth. One cubic yard spread evenly covers:
| Depth | Area covered |
|---|---|
| 1 in | 324 ft² |
| 2 in | 162 ft² |
| 3 in | 108 ft² |
| 4 in | 81 ft² |
Each row is 324 ÷ depth; the 1 in row is the figure Clemson states.
How much mulch for 100 square feet?
At 3 in deep: 100 × 3 ÷ 12 = 25 ft³, which is 0.93 yd³, or 13 bags of 2 cu ft (12.5 rounded up). For 200 ft² double it: 50 ft³, 1.85 yd³, 25 bags.
How deep should mulch be to prevent weeds?
The two extension guides we cite put it slightly differently. University of Missouri Extension gives 2 to 4 in for most mulches, for insulating the soil as well as for weeds (G6960). Clemson: “A 2- to 3-inch layer of wood chips provides good weed control” (HGIC 1604). Clemson also sets a ceiling, and the reason is roots, not weeds: fine wood and bark mulches “should not be more than 2 to 3 inches deep,” because “Excessive amounts of these fine-textured mulches can suffocate plant roots”; coarse ones such as pine bark nuggets “can be as deep as 4 inches.” We don’t pick one for you.
Should mulch touch the tree trunk?
No. Clemson: “Keep mulch 2 to 3 inches away from the stems of woody plants” and “do not pile mulch against the trunk.” That is why the Ring shape has a bare circle box.
What this calculator is not for
It doesn't pick a mulch type or tell you how deep a particular plant wants it. The two guides cited below give general ranges: Missouri 2 to 4 in for most products; Clemson no more than 2 to 3 in for fine wood and bark mulches and up to 4 in for coarse ones. Check your local extension office for your plants and climate. Fresh mulch also settles, and we found no published figure for how much, so what you spread may not stay at the depth you measured.
Sources
- University of Missouri Extension, Mulches (G6960). Checked 2026-09-24.
- Clemson Cooperative Extension, Home & Garden Information Center, Mulch (HGIC 1604, revised June 20, 2016). Checked 2026-09-29.
“A 2- to 3-inch layer of wood chips provides good weed control. … Keep mulch 2 to 3 inches away from the stems of woody plants. This will prevent decay caused by wet mulch and rodent damage during the winter. … Newly planted trees require a circle of mulch 3 to 4 feet in diameter; however, do not pile mulch against the trunk. For established trees in lawns create a circle of mulch about 2 feet in diameter for each inch of trunk diameter. … Many wood and bark mulches are composed of fine particles and should not be more than 2 to 3 inches deep. Excessive amounts of these fine-textured mulches can suffocate plant roots, resulting in yellowing of the leaves and poor growth. … Coarse-textured mulches such as pine bark nuggets allow good air movement through them and can be as deep as 4 inches. … There are 27 cubic feet in a cubic yard. One cubic yard will cover a 324-square-foot area with an inch of mulch.”
- NIST, NIST Handbook 44 (2026), Appendix C — General Tables of Units of Measurement. Checked 2026-09-24.
“1 728 cubic inches (in3) = 1 cubic foot (ft3) 27 cubic feet = 1 cubic yard (yd3)”