Topsoil is often added to a garden to make the soil more loamy and rich, which is ideal for most plants to thrive. Also, topsoil is sometimes added along with compost or another organic matter, such as peat moss. Since topsoil and other related materials are sold in cubic feet or cubic yards, you must calculate how much you need in those units of measurement.
Figure out the square footage (in feet) of the area where you want to add topsoil. For rectangular areas, multiply length and width. For circular areas, multiply pie (3.1416) and the radius squared. For triangular areas, multiply ½ times base times height.
Decide how deep you want the topsoil to be. Convert it into feet. For example, if you want to add 3 inches of topsoil, this equals 0.25 feet (3 divided by 12).
Calculate how much cubic feet of soil you need. Multiply the square footage from Step 1 and the depth in feet from Step 2. If the package is sold in cubic yards, divide this number by 27.