How much compost you may need depends on the depth you will lay it down and that can be calculated by multiplying Width by Length to get your total area and then multiplying that by the Depth you will be spreading that compost. Width and Length can be in feet and that results in square feet and times Depth, in feet (or fractions of feet, ie. 1/12 if the depth will be one inch) will give you the cubic feet of material needed.
So a space 10 feet by 10 feet would be 100 square feet times 1 inch depth (1/12 or 0.083) equals 8.3 cubic feet of material. You can go further and divide the cubic feet by 27 (the number of cubic feet in one cubic yard) to get the number of cubic yards needed.
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