To calculate the mulch quantity, calculate the area of the garden, reduce it by 10 percent, and determine how deep the mulch should be. Then, divide the reduced garden area by 320, 160, 110 or 80 depending on the mulch depth required.
To calculate the area of the garden, measure the length and width of its bed in feet, and multiply the two measurements together to arrive at the garden area in square feet. If the garden has a circular shape, measure the radius of the circle, square the figure, and multiply the same by a factor of 3.14.
Remove 10 percent of the garden area from your calculations. For example, if the garden area is calculated to be 100 feet, subtract 10 feet from it. This accounts for mulch displacement by shrub and plant roots.
Normally, 3 inches of mulch is adequate for a garden bed. If this is the required mulch depth, divide the reduced garden area by 110. If 1-inch mulch depth is required, divide by 320. Similarly, divide by 160 if mulch of 2-inch depth is required and by 80 for a mulch depth of 4 inches.
The result arrived at is the quantity of mulch required in cubic yards. To convert the result to cubic feet, multiply by 27.