How Big Should the Backing Be?
Fabric is taken up during the quilting process and therefore requires that the backing be larger than quilt top. This extra fabric also allows for any adjusting in the case that the quilt top is not perfectly squared.
What dimensions should the backing be?
1. Measure the width (x) of the quilt top and add 8 inches. x + 8 = W.
2. Measure the length (y) of the quilt top and add 8 inches. y + 8 = L.
Your backing dimensions are W x L. (and you thought you'd never use algebra)
If your fabric width is equal to or larger than W, divide L by 36 and that is how many yards you need.
If the fabric width is smaller than W, divide W by the width of fabric (WOF), round up to the next whole number. Multiply that times L. Divide by 36 for the number of yards needed. You may have extra fabric but this allows for minimal seams in the backing and smooth stitching on the top.