Topsoil is, as the name states, soil placed on top of an area. Topsoil is often used as a medium in gardens to plant seeds. It has most of the nutrients needed for proper growth of the plants. The amount of topsoil required depends on the size of the area and the application. Using simple multiplication and division you can determine how much topsoil you need for your project.
Determine the depth of the topsoil requirements for your needs. Sod requires a depth of 3 to 6 inches according American Lawns , Gardening Data suggests grass seed uses a depth of 4 to 8 inches and vegetables mostly require a depth of 8 to 12 inches according to Oasis Farms.
Measure the total length and width of the area being covered with the measuring tape. Use decimal fractions for each portion of 1 foot for easier calculations--for example, a 6-inch measurement would be 0.5 foot. Multiply the length by the width to get the total square footage of coverage. For odd shaped lots, break the area down into smaller squares or rectangles; measure the length and width of each, find the total and then add the totals to find the end total for the entire area.
Find the diameter of the circle (for circular plots) by measuring the radius and multiplying the total by 3.14 to get the complete area of the circle. Find the radius by measuring from any edge of the circle to the center of the circle.
Multiply the total area to be covered by the required number of inches of topsoil to get the total cubic feet needed for the area. For a 5-by-10 foot area you would multiply 5 by 10 to get the total square feet covered--50 square feet. Then multiply 50 square feet by 0.5 foot (assuming that you need 6 inches of topsoil, for planting sod) and you will find out that you need 25 cubic feet of topsoil.
Convert the total cubic feet to yards because topsoil is sold in cubic yards. Divide the cubic feet by 27 to get the cubic yards. For example, using the total from above, 25 divided by 27 would yield 0.925 cubic yards.
Round the total cubic yards up or down to the nearest whole number; using the example this would make 1 cubic yard. This ensures enough topsoil for the project.