Use 108 inch extra wide quilt backing for a seamless quilt back. Using quilt cotton backing fabric for quilts gives your quilt a seamless look.
Most patterns will tell you how many yards of 108 inch fabric is needed for quilt backing. However, if you measure with plus length that will tell you how many yards is needed. The fabric is 108 inches, then you just need to know how many yards of quilt backing is needed for the length.