At the risk of forcing flashbacks from high school math class, there is a formula you can use to determine how much soil you need. First, figure out the square footage of the beds. To do this, multiply their length in feet by their width in feet. Then determine how deep you want the soil in the beds, convert it to feet (divide by 12 because there are 12 inches in a foot), and multiply that number by the square footage. This will give you cubic feet. Divide that number by 27 to get the number of cubic yards you'll need, because soil is usually sold by the cubic yard. As an example, if you want to add a 10X8-foot bed, multiply 10 and 8, to get 80 square feet. If you want the soil depth to be 6 inches, multiply 80 by 0.5 (because 6 inches is half a foot or 6/12), to get 40 cubic feet. Divide that by 27 and you'll find you need about 1.5 cubic yards of soil.