How it's calculated
Mulch is sold by volume: in bags marked in cubic feet (or liters), or in bulk by the cubic yard (or cubic meter). The calculator multiplies the bed's area by the depth you want, converts that to cubic yards, and counts the bags it would take, so you can compare bagged and bulk prices.
Clemson Cooperative Extension gives the rule directly: one cubic yard covers 324 square feet at a depth of 1 inch, so multiply the square footage by the depth in inches and divide by 324 to get cubic yards Clemson Extension. The 324 is simply 27 cubic feet in a cubic yard times 12 inches in a foot.
Depth matters for the plants as much as for the budget. The same Clemson fact sheet says fine-textured wood and bark mulches should be no more than 2 to 3 inches deep, coarse mulches such as pine bark nuggets can go to 4 inches, and grass clippings or shredded leaves should never be deeper than 2 inches, because they mat down and starve roots of air and water. The calculator starts at 3 inches.
The formula
- Area = bed length × bed width (square feet).
- Cubic yards = area × depth in inches ÷ 324, plus any extra allowance you add.
- Cubic feet = cubic yards × 27.
- Bags = cubic feet ÷ bag size in cubic feet, rounded up. Bag cost = bags × price per bag.
- Bulk cost = cubic yards × price per cubic yard (delivery fees are extra).
- Metric: area in m², depth in cm and bags in liters are converted with NIST's exact factors (1 ft = 0.3048 m, 1 in = 2.54 cm) NIST, and results are shown in cubic meters and liters.
Worked example
A 10 × 20 ft bed mulched 3 inches deep needs 1.85 cu yd of mulch, which is 50 cu ft. In 2 cu ft bags that is 25 bags, costing $100 at an example $4 a bag, against $74 in bulk at $40 a cubic yard.
Mulch needed: 1.85 cubic yards
At 2 inches, the depth Clemson gives as the most for grass clippings or shredded leaves, the same bed needs 1.23 cu yd; at 4 inches of coarse pine bark nuggets it needs 2.47 cu yd.
Assumptions and limitations
- Rectangular beds. For round or curved beds, find the area with the square footage calculator and enter an equivalent length and width.
- Settled vs fresh depth. Fresh mulch settles. If you are topping up an old layer, measure what is there and add only the difference.
- Bag size is on the bag. The 2 cu ft default (50 L in metric) is only an example; bags come in several sizes.
- No delivery fee. Bulk prices usually exclude delivery, which can make bags cheaper for small beds.
- Keep mulch off stems and trunks. Clemson advises keeping mulch 2 to 3 inches away from the stems of woody plants and 6 to 12 inches from building walls.
- Prices are examples. Enter your own.
Spreading gravel or stone instead? The gravel calculator also converts volume to tons.