Topsoil is added to a lawn to fill in low areas or to prepare the lawn for adding grass sod or seed. Because topsoil is sold in cubic yards or cubic feet, calculating the amount of topsoil needed for adequate coverage can be challenging.
Calculate Yard Size

Measure the length and the width of the lawn in order to calculate the area's square footage. If the lawn is irregularly shaped, separate the lawn into smaller squares and rectangles, and then write down the length and width of each of the smaller areas.
The square footage of each section is calculated by multiplying the length by the width of each area. For example, if the length of one section is 20 feet long and the width is 10 feet, then the square feet is calculated as follows: 20 feet x 10 feet = 200 square feet.
Cubic Feet Multiplier

Because topsoil is sold by the cubic yard or feet, you will need to figure out a multiplier to convert the amount needed in cubic feet then convert to cubic yards. To do this, divide the desired depth of the topsoil by a factor of 12 to get the needed multiplier to find out how many cubic feet you need.
For example, if you would like the topsoil to be added to a depth of 3 inches, then 3 divided by 12 = 0.25, which is the cubic feet multiplier.
Calculate Cubic Feet

Now that you have established the multiplier needed to convert the cubic feet of soil needed to fill the square feet area to a depth of 3 inches, multiply the yard's square footage by the multiplier. In this example, it would be 200 X 0.25 = 50 cubic feet.
Convert Cubic Feet to Cubic Yards

Because bulk topsoil is sold in cubic yards instead of cubic feet like bagged topsoil, you need to convert the amount of cubic feet to cubic yards by dividing the amount of cubic feet by 27.
For example, if the amount of cubic feet needed is 50, then 50 divided by 27 is 1.85 cubic yards.
References
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