you need to know the finished size of your quilt. And for those who are math challenged, use some graph paper. For the backing, use 4 squares for a yard (each square would represent 9" or a quarter yard). Mark off a 108" wide (3 yards) and draw a long way down (open ended). Use a pen or a sharpie and you can keep reusing this template. The with a pencil, mark in the size of your finished quilt. This will let you see how much you will need. You can also pencil in what you would need for binding if you are using it for that also. This is a bit cumbersome, but many people have an easier time when they can see it actually written down.