If you are backing your quilt with vertical orientation of the fabric, the calculation is easy: If the width is less than 40", the length equals yardage. If the width is less than 80", then 2x length, etc. It is important that no seams fall where the finished quilt will be folded, to ensure that no stress points fall on the fold line. You'd hate to put all the work in to the quilt top and quilting and have it pull apart at the backing seams! 

There are situations where aligning the backing fabric horizontally makes better use of fabric, and makes a better quilt. The Quilter's Backing Wizard is a great look-up tool to tell you how much backing fabric you will need if aligning the backing fabric horizontally. 

The Quilter's Backing Wizard is a great look-up tool to tell you how much backing fabric you will need if aligning the backing fabric horizontally. 

The mathematical assumptions are clearly described on each wizard. All you have to do is measure the length and width of your quilt top.