When planning your growing area, you will need to know the number of cubic yards of topsoil to lay in your garden. The amount of topsoil you use to cover your garden will depend on whether you wish to simply lay out a thin 1- or 2-inch layer or build a 9- to 12-inch-deep raised bed. Once you know the yards of topsoil, you use that number to purchase the correct amount for your garden.
Measure in feet the length and the width of the space you will fill with topsoil.
Multiply the length by the width to find the area of a rectangular space. For instance, for a rectangular garden patch 10 feet by 12 feet: 10 x 12 = 120 square feet.
Find the area of a circular garden by multiplying the diameter of the space by itself and multiplying the result by 3.14. For example, if you have a circular lawn of 10 feet in diameter: 10 x 10 = 100 x 3.14 = 314 square feet.
Determine the area of a triangular garden by multiplying the length by the width by 1/2. For example, if you're working on a triangular garden 10 feet long by 12 feet wide: 10 x 12 x 1/2 = 60 square feet.
Divide the planned depth of the topsoil in inches by 12 to determine the measurement in feet. For example, if you wish to lay a 9-inch-deep layer of topsoil: 9/12 = 3/4 feet.
Multiply the area in square feet by the depth of the topsoil in feet to find the cubic feet volume for the topsoil. For example, if you had a rectangular garden measuring 10 feet by 12 feet with a topsoil depth of 9 inches: 10 x 12 x 9/12 = 90 cubic feet.
Divide the cubic feet by 27 to find the number of cubic yards of topsoil. For the example: 90/27 = 3.333 cubic yards.