Calculate how much topsoil you need by multiplying the garden area's dimensions in square footage (length x width) by the depth of topsoil needed to get cubic footage needed for topsoil. 2 inches is the layer of topsoil you generally want to spread over the garden bed. So, if you have a garden that's 2 feet by 2 feet, that is 4 sq ft. or 576 square inches. Multiply 576 x 2 inches for 1152 cubic inches of topsoil needed. 1152 cubic inches is equal to 0.67 cubic feet (1152/1728), so that is the amount of topsoil needed.