Backing yardage for a project, piecing widths as needed.
Pieces full widths horizontally when the project is wider than the fabric.
The backing of a quilt must be bigger than the top, so that the layers have room to shift while they are quilted and the edges can be trimmed square. A common allowance is two to four inches on each side. Most quilts are wider than the 42 to 44 inches of usable fabric on a bolt, so the backing usually needs to be pieced from two or more lengths sewn together.
The calculator works out how many widths you need and the total yardage. Running the seams lengthwise is the usual choice, but a quilt that is wider than it is long may use fewer yards with the seam running across. Compare both layouts before you buy.
Wide backing fabrics that are 90 to 108 inches wide avoid a seam altogether and are often worth the higher price per yard. Trim the selvages from the edges before joining pieces, since they are tightly woven and can pucker when quilted.
Pre wash the backing if you pre wash the top, so that both shrink alike. Choose a print that hides stitching if you are not confident about your quilting, and press the seam open so the quilting passes over it smoothly.