The amount of mulch you need will primarily depend on the size of your flowerbeds. A good rule of thumb is to maintain a depth of three inches. To calculate the amount of mulch you require, measure the approximate square footage of your garden. Multiply the total square footage area of your yard by the depth required. Divide the result by 324.
So, for example to calculate the total square footage area of your lawn is 500 feet and the required depth is 3 inches, you will need 500*3/324 = 4.62 cubic inches of mulch.