To figure out how much topsoil you need, you need to multiply length x width x depth, and then divide by 2. For example, if the soil is sunken down 4 inches (1/3 foot) at your house’s (foundation length of 100 feet), and assuming you need to grade the soil 4 feet out from the foundation, then you need to multiply your measurements together like this: .3333 feet x 100 feet x 4 feet = 133.2 cubic feet. But because you are grading the soil out 4 feet into the yard at a depth of zero, the average depth would be half of .3333 feet. To get the correct answer, divide 133.2 in half, giving you the actual soil needed, 66.6 cubic feet. To turn this into cubic yards, you need to divide 66.6 by 27 (since there are 27 cubic feet in one cubic yard (3 x 3 x 3 = 27). The amount of topsoil you will need for this example is 2.46 cubic yards.