NET AREA → EXACT VOLUME
Mulch Coverage Calculator
For a bed measured in square feet, cubic yards needed = area × depth in inches ÷ 324. This calculator first removes a circular spot you will not mulch—such as a tree trunk area—then shows the exact volume and what that volume can cover at another depth.
12 FT × 9 FT AT 3 IN
Use 1 cubic yard for 108 sq ft
The rectangular bed is 108 sq ft. At 3 inches, 108 × 3 ÷ 324 = 1 cubic yard.
What this mulch coverage calculator answers
Coverage is the net surface you will mulch, not always the full outside rectangle. Start with length × width. If a circular spot stays bare, subtract π × (diameter ÷ 2)². Then multiply the remaining square footage by the depth and divide by 324 to get cubic yards.
Why 324 is in the formula
One cubic yard is 27 cubic feet. At one inch deep, that volume covers 27 × 12 = 324 square feet. Every additional inch of depth divides that coverage: at 3 inches, one yard covers 324 ÷ 3 = 108 sq ft.
| Depth | Coverage from 1 cubic yard | Exact calculation |
|---|---|---|
| 1 inch | 324 sq ft | 324 ÷ 1 |
| 2 inches | 162 sq ft | 324 ÷ 2 |
| 3 inches | 108 sq ft | 324 ÷ 3 |
| 4 inches | 81 sq ft | 324 ÷ 4 |
Reverse-check an order before delivery
Once you have a cubic-yard quantity, you can reverse the calculation instead of trusting a rounded label: coverage sq ft = cubic yards × 324 ÷ depth in inches. For example, 2 cubic yards at 3 inches cover 2 × 324 ÷ 3 = 216 sq ft. This checks whether an order quantity matches the measured net area; it does not decide how a supplier requires orders to be rounded.
A circle in a rectangular bed: worked example
A 12 ft × 9 ft bed is 108 sq ft. If a 2 ft-diameter circle will not receive mulch, its area is π × 1² = 3.14 sq ft. Net coverage is 108 − 3.14 = 104.86 sq ft. At 3 inches: 104.86 × 3 ÷ 324 = 0.97 cubic yards. Keep the unrounded calculation separate from the supplier’s order increment.
Coverage mistakes to avoid
- Using the outer rectangle when paths, pots, trunks, or islands will stay bare.
- Using the one-inch figure of 324 sq ft for a three-inch layer; the three-inch figure is 108 sq ft per yard.
- Rounding every small bed before adding them. Add net areas first, calculate one total volume, then check the supplier’s ordering increment.
- Mixing feet and inches without converting depth. The ÷324 shortcut works only for square feet, inches, and cubic yards.
FAQ
Can I calculate a circular mulch bed?
Yes. For a full circle, use π × (diameter ÷ 2)² for area, then apply area × depth ÷ 324. The main Mulch Calculator has a circle input for that direct estimate.
How much does one yard of mulch cover?
It depends on depth. At 3 inches, one cubic yard covers exactly 108 sq ft; see the full one-yard coverage table and reverse check.
How many bags equal a cubic yard?
One cubic yard is 27 cubic feet. That is 13.5 bags of 2 cubic feet, so 14 whole bags provide at least one yard. Use How Many Bags of Mulch Do I Need? for a bag calculation.
Related reading: Pine Straw Calculator — calculate whole bales from the coverage stated for the exact product you are buying.