Mulch is anything spread over the surface of the soil in a garden to control weeds and conserve moisture. However, most gardeners think of plastic bags filled with bark chips or shredded wood when they think of mulch. Mulch works best when spread to a depth of 3 to 4 inches, because it breaks down or decays quickly once applied, especially in warm weather. When thinking of covering a garden with mulch, you can calculate exactly how much to buy by following a formula.

Calculate the square foot area of the flower bed or garden. Do this by measuring two sides of the desired area to be mulched and multiplying the two sides. For example, if one side of the garden is 20-feet-long and another is 10-feet-long then 10 x 20 = 200, so the square footage is 200 square feet.

If the garden is odd-shaped then mark it off in squares calculating the square foot of each marked off square and adding them together to get the total square feet.

Calculate how many cubic yards you will need based on the desired depth of the mulch. For example, if you want the mulch to be 4 inches deep when applied use the following formula:

200 (area to cover in sq ft) x 4 (depth of mulch in inches) x .0031 (amount of cubic yards needed per linear foot--this amount remains constant and is the same in all calculations) = 2.48 (cubic yards of mulch required)

Convert the cubic yards needed to cubic feet because bags of mulch are sold by cubic feet, usually 2 or 3 cubic feet in each bag. There are 27 cubic feet in a cubic yard.

2.48 (cubic yards required) x 27 (number of cubic feet per cubic yard) = 66.96 (cubic feet needed)

Calculate the number of bags needed. If a bag contains 2 cubic feet and you need 66.96 cubic feet then divide the number of cubic feet needed by 2.

66.96 (cubic feet required) / 2 = 33.48, or 34 bags of mulch needed for project