I usually buy enough to cover the smallest measurement of the quilt I am backing. Add in 8" if you are having it longarmed. For example say a quilt is 75" x 85". I would buy 2.25 yards if it were being longarmed. You would have a piece 80" x 20" left. But the idea of using the large backing is to not have to piece the backing. ;o) (If it matched, you could use it for binding)