To calculate the cubic yards equal to 500 square feet, first determine the depth of mulch or other substance, explains Green Seattle Partnership. Multiply that measurement, in feet, by 500 square feet to determine the number of cubic feet. Divide by 27, since 1 cubic yard equals 27 cubic feet.
For instance, if 6 inches of mulch is desired, 500 square feet times 0.5 -- one-half of a foot -- equals 250 cubic feet, Green Seattle Partnership recommends. Divide 250 cubic feet by 27 cubic feet, and the result is approximately 9.26 cubic yards. The equations vary depending upon the depth measurement, which must always use the "foot" unit.