The Topsoil Calculator will help you figure out how much topsoil or potting soil to buy, with a breakdown in terms of bags or yards. It will convert your measurements into cubic feet and yards so you'll know how much potting soil you'll need for your window box, garden plot, flower bed, or yard. Just supply the length, width and depth of your project.
One cubic yard equals 27 cubic feet.
A 40 pound bag of topsoil usually contains about .75 Cubic Feet of soil.
There are 25.71404638 Dry Quarts in a Cubic Foot, so a 25 quart bag of potting soil would equal approximately 1 Cubic Foot.