First, calculate the square footage by multiplying the area's length by its width.  If you plan to add 3 inches of organic material, multiply your square footage by .25 (3 inches is 25% of 1 foot).  Divide that number by 27 for cubic yardage.

A 3.8 cubic foot bale of peat moss will cover a 100 square foot area, so if your new bed is 300 square feet, you'll need 3 bales.

Got this info from the current issue of Country Woman Magazine