How much bigger should the backing be?
Longarm quilters usually want 4 in extra on every side (8 in added to each dimension) so the backing can be clamped to the frame. For quilting on a home machine, 2 to 3 in per side is enough.
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Quilt backing and binding calculator
Enter your finished quilt size and who's quilting it. You'll get backing yardage both ways (with the seam check most calculators skip), wide-back yardage, binding strips and a batting size.
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Assumes 40 in usable width of fabric and a 1/2 in seam allowance on each side of a backing seam. Rounded up to the next 1/8 yd. Prewash or not, buy from one bolt.
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Good to know
Piecing the backing
Most quilting cotton gives about 40 in of usable width, so anything wider needs pieced widths. Each seam uses about 1 in, so two widths cover 79 in, not 80. That's why this calculator checks coverage before suggesting a layout, and compares running the seam each way.
Wide backing
108 in wide backing avoids seams. If the backing fits across the width, you only buy the other dimension, often less yardage than pieced cotton.
Binding
Double-fold binding strips are usually 2-1/4 or 2-1/2 in wide. You need the quilt's perimeter plus about 12 in for corners and joining.