How much backing do I need?
The backing must be larger than the quilt top so it can be loaded on a frame or basted without coming up short. Add the extra to every side:
backing size = (top width + 2 × extra) × (top length + 2 × extra)
Unless the quilt is narrow, standard 40–42" fabric has to be pieced. You can run the seams vertically (panels as long as the quilt) or horizontally (panels as long as the quilt is wide). The calculator works out both, accounting for ½" of seam allowance per join, and picks the one that needs less fabric.
Example
A 60" × 72" top with 4" extra on every side needs a 68" × 80" backing. From 40" fabric, two vertical panels of 80" are enough: 160", or 4½ yards. Horizontal seams would need three panels of 68" (5¾ yards), so vertical seams win here. For other sizes it is often the other way around.
Wide-back fabric
Extra-wide backing (around 108") avoids seams for most quilts. Enter its usable width above to see the yardage. It often costs more per yard but you buy much less.
Frequently asked questions
How much extra backing do I need for longarm quilting?
Most longarm quilters ask for 4" to 6" extra on every side, so 8"–12" wider and longer than the top. Check with your quilter before buying.
Should backing seams be vertical or horizontal?
Either works. Choose whichever uses less fabric (the calculator shows both). Some longarm quilters prefer seams running horizontally; ask if you are sending it out.
How much backing for a queen quilt?
A 90" × 108" top with 4" extra on each side needs a 98" × 116" backing. With 40" fabric that is three horizontal panels, about 8¼ yards; with 108" wide backing it is about 3¼ yards.