Enter the quilt size and how much extra you want on each side. The calculator tries panels running lengthwise (vertical seams) and crosswise (horizontal seams), and shows the layout that uses less fabric, with the panel length and the yardage to buy.
Worked example: backing a throw quilt
A 50 x 65 in throw with 4 in of extra backing on every side.
- Backing size: 50 + 8 = 58 in wide, 65 + 8 = 73 in long.
- Lengthwise panels: (58 - 1) / (42 - 1) rounds up to 2 panels of 73 in = 146 in of fabric.
- Crosswise panels: (73 - 1) / (42 - 1) rounds up to 2 panels of 58 in = 116 in of fabric.
- Crosswise uses less, so cut 2 lengths of 58 in and join them with 1 horizontal seam.
- 116 in / 36 rounds up to 3 1/4 yd.
Result: 3 1/4 yd of 42 in fabric, 2 panels, 1 horizontal seam.
How backing yardage is calculated
- Backing width = quilt width + 2 x overhang. Backing length = quilt length + 2 x overhang.
- Each join is sewn with a 1/2 in seam, which uses 1 in of the panels' combined width, so n panels cover n x usable width - (n - 1) in.
- Lengthwise layout: panels = (backing width - 1) / (usable fabric width - 1), rounded up; each panel is as long as the backing. Fabric = panels x backing length.
- Crosswise layout: panels = (backing length - 1) / (usable fabric width - 1), rounded up; each panel is as long as the backing is wide. Fabric = panels x backing width.
- The calculator keeps whichever layout needs less fabric (on a tie, the one with fewer seams).
- Yards = fabric length / 36, rounded up to the next 1/8 yard.
Assumptions
- Overhang is 4 in on each side by default. If someone else is quilting it, ask them: longarm quilters often want 4 to 6 in, and some want more.
- Usable width of fabric is 42 in after trimming selvages. For extra-wide backing fabric, enter its usable width instead.
- Panels are joined with a 1/2 in seam, pressed open; the calculator subtracts that 1 in per join.
- Panels are full width; the calculator does not plan a pieced back from leftover scraps.
- No allowance for shrinkage if you pre-wash the backing.
How to piece a quilt backing
Join panels of 42 in fabric into a backing 8 in larger than the quilt top in each direction.
- Use the calculator to find the number of panels and the length to cut each one.
- Cut the panels to length, then trim the selvages off the long edges that will be joined.
- Pin the panels right sides together along the trimmed edges and sew with a 1/2 in seam.
- Press the seam open to reduce bulk, then square up the backing and check it is larger than the top on every side.
Frequently asked questions
How much bigger should the backing be than the quilt top?
A common rule is 4 in larger on every side, so 8 in wider and 8 in longer. Longarm quilters often ask for 4 to 6 in per side, so check with whoever is quilting it.
How much backing do I need for a queen quilt?
For a 90 x 108 in queen with 4 in overhang, the backing is 98 x 116 in. With 42 in fabric that is 3 panels of 98 in, 8 1/4 yd in total.
Should backing seams run vertically or horizontally?
Either works structurally. Horizontal (crosswise) seams often use less fabric on throws and wide quilts; vertical seams can be easier to keep straight on a longarm frame. The calculator shows the layout that uses less fabric.
Can I use extra-wide backing fabric?
Yes. Change the usable width to the width of your wide-back fabric after trimming selvages. If the backing fits in one width, you only buy one length and skip the seam.