- 1). Measure the length and width of the area that needs topsoil. If the space is not uniform in shape, use the largest length or width to ensure you purchase enough.
- 2). Convert any inches measurements to feet by dividing the measurement by 12. For example, a 3-foot-10-inch length, or 46 inches, converts to 3.83 feet. Multiply the length in feet by the width in feet to find the square-footage of the space.
- 3). Factor in the depth of the topsoil. Divide the inches of depth by 12 to convert it to feet. Then multiply the square-footage of the area by the depth in feet to find the cubic feet of topsoil needed. Round the answer up to a whole number, if needed.
- 4). Shop at local home and garden centers to see how much various brands and packages of topsoil cost per cubic foot. If the package holds 2 cubic feet, for example, divide the price per package in half to determine the cost of 1 cubic foot of topsoil.
- 5). Choose the brand of topsoil you want to use. Multiply its cost per cubic foot by the number of cubic feet in your project to find the full cost of the topsoil needed. Take into account the size of the packaging as you make your calculations. For example, if you are considering soil in a 2-cubic-foot package, and you need 5 cubit feet, then you must find a place to store or use the extra topsoil.