Calculate the amount of topsoil needed for a given area by multiplying the length by the width by the desired depth. Topsoil is generally sold by the cubic foot, so this calculation should reveal how much to buy. The necessary depth of topsoil depends on the project, but around 4 inches is usually sufficient. HomeDepot.com features a mulch and topsoil calculator that helps with this process.
Filling a garden bed typically requires significantly more topsoil than other projects. Six to 8 inches is usually recommended for this type of project. Planting a new lawn usually requires less. Generally, between 4 and 6 inches of topsoil is sufficient for a lawn.
The easiest method for measuring the length and width of a garden bed is to place the measuring tape's metal hook, called a tang, into the corner of the plot and have a helper hold it there. Many plots are not rectangular. To measure these, break the area down into a series of rectangles and add their areas. This is not a perfect system, but is the simplest way to get a good ballpark number.
When calculating the amount of topsoil needed, make sure to use consistent units. If using a length and width measured in feet, divide the number of inches of topsoil by 12. For example, 6 inches of topsoil equals 0.5 feet of depth.