To calculate how much mulch is necessary for a project, first determine the square footage of the area and the thickness of the mulch, subtract the area with plant coverage, and then convert the square footage to cubic feet, according to About.com. Mulch is typically available in bags by cubic volume or weight.
To measure the square footage of the area, multiply the length by the width, explains About.com. If the area is a triangle shape, measure the sides that form a right angle, and then multiply the length of one of the sides by 1/2 the length of the opposite side. If the area is in circular shape, measure the radius of the area, multiply this by itself, and then multiply the resulting number by 3.14 to get the square footage.
After determining the square footage, approximate the area that has plant coverage, and subtract this, states About.com. Estimate how many inches of mulch are necessary, and then convert this number into cubic yards, since mulch is usually available by volume. There are 27 cubic feet in a cubic yard, and an average 40-pound bag is equal to about 1 cubic foot. Keep in mind that these numbers are estimates, but with these calculations, the estimate is close and should ensure coverage of the area.