How to Plan and Piece a Quilt Backing
Size a quilt back with the right overhang, decide how many fabric widths to seam, and choose between standard and extra-wide backing.
Start with the backing size, not the quilt size
A quilt back must be larger than the quilt top on every edge so the layers can be basted, quilted and trimmed square. Begin by adding your chosen overhang to both sides of each dimension, which means adding twice the overhang to the width and twice to the length. A 60 by 80 inch top with a 4 inch overhang therefore needs a backing that finishes at least 68 by 88 inches. That extra border gives you room to load a longarm frame or to smooth out the layers on a table without running short at a corner.
Counting the fabric widths you must seam
Standard quilting cotton gives about 44 inches of usable width after you trim the selvages, so any backing wider than that has to be pieced from full widths joined side by side. Divide the backing width by the fabric width and round up: 68 divided by 44 is 1.5, which rounds up to 2 widths. A wider quilt can need 3 or more widths, and each added width multiplies the yardage because you buy the full backing length for every panel. The calculator does this rounding for you so you never end up one panel short.
Turning widths into yards
Once you know how many widths you need, the yardage is simply those widths multiplied by the backing length, then divided by 36 inches per yard. For a backing that is 88 inches long and needs 2 widths, that is 176 inches of fabric, or about 4.8889 yards. A three-width king backing 116 inches long climbs to roughly 9.6667 yards, which shows how quickly wider quilts eat fabric. At the shop, round that figure up to the next quarter or half yard and add a little extra for prewash shrinkage and for trimming the ends straight.
When wide backing is worth it
Extra-wide backing is woven at 108 inches so it can cover almost any bed quilt in a single piece with no center seam. Setting the fabric width to 108 in the calculator usually drops the widths needed to 1, which both saves fabric and removes the bulk of a pieced seam under the quilting. Wide backing often costs more per yard, so compare the total cost against two or three widths of standard cotton. For large quilts the wide goods frequently win on both price and on a smoother, seam-free back.