I just used it to calculate some yardages for a project I'm working out (am considering a Hunters Star quilt). If my math is accurate, then the calculator is accurate. So if I were purchase exactly what it says I need for all my shapes, I should have exactly enough for my quilt (I plan to purchase extra just incase, and I would love to have left overs any way).
You select the shape of the piece you need (choices are square, rectangle, HST, HRectangeT, QST, trapezoid and parallelogram), enter it's size (finished or cut), how many you need, and WOF. It then gives you a page that states the shape, how many pieces you entered, both cut and finished dimensions, and cutting directions along with the required yardage (appears to be rounded up to the nearest 8th yd). The cutting instructions tell you how many strips of what size to cut, including your WOF entered. It then tells you how many pieces from each strip and if there are any spare it will tell you how many extra you end up with. I was really impressed (can't you tell)!!
NAYY, just really liked and thought I'd share.