How much backing fabric do I need?
The backing has to be bigger than the quilt top: usually 4 in (10 cm) extra on every side if a longarm quilter is finishing it, and 2–3 in if you quilt at home. So a 50 × 65 in throw needs a backing of 58 × 73 in.
Standard quilting cotton is only about 40 in wide once the selvedges are trimmed, so most backings are pieced from two or more panels. You can join them side by side (vertical seams, each panel as long as the quilt) or one above the other (horizontal seams, each panel as long as the quilt is wide). One way often uses a yard or more less fabric than the other. The calculator works out both and picks the cheaper layout.
Backing yardage for common quilt sizes
With 4 in extra on each side, rounded up to the next 1/4 yard.
| Quilt | 42 in cotton | 108 in wide back |
|---|---|---|
| Baby / crib, 36 × 52 in | 2 1/2 yd | 1 1/4 yd |
| Throw / lap, 50 × 65 in | 3 1/4 yd | 1 3/4 yd |
| Twin, 70 × 90 in | 5 1/2 yd | 2 1/4 yd |
| Full / double, 85 × 108 in | 7 3/4 yd | 3 1/4 yd |
| Queen, 90 × 108 in | 8 1/4 yd | 3 1/4 yd |
| King, 108 × 108 in | 9 3/4 yd | 6 1/2 yd |
The formula in plain English
- Backing size = quilt width + 2 × extra, by quilt length + 2 × extra.
- Panels = backing width ÷ usable fabric width, rounded up (allowing 1/2 in for each seam). Try it both ways round.
- Fabric to buy = panels × panel length, rounded up to 1/4 yard (or 10 cm).
- Batting = the same size as the backing, cut from a roll wide enough to need no joins.
- Binding = the quilt's perimeter + 10 in, divided by 40 in per strip, times the strip width.
Worked example: a 50 × 65 in throw
- Backing size with 4 in extra each side: 58 × 73 in
- Vertical seams: 58 in needs 2 panels of 40 in fabric, each 73 in long = 146 in (4 1/4 yd to buy)
- Horizontal seams: 73 in needs 2 panels, each 58 in long = 116 in (3 1/4 yd to buy)
- Horizontal wins: buy 3 1/4 yd, a whole yard less
- Batting from a 96 in roll: 58 in long, so 1 3/4 yd (or a twin size pack)
- Binding: 2 × (50 + 65) + 10 = 240 in, so 6 strips of 2.5 in = 1/2 yd
- At $13/yd for cotton and $14/yd for batting: $42.25 + $24.50 + $6.50 = $73.25 of fabric
Wide backing or pieced?
Wide backing fabric (108 in, about 275 cm) covers quilts up to queen size in one piece, with no seams to line up. It costs more per yard but you buy much less, so for large quilts it's often the cheaper option as well as the easier one. A king quilt, at 108 in or wider, is bigger than one width of wide backing, so it still needs a seam. Choose it from the fabric width list to compare.
FAQs
How much bigger should the backing be than the quilt top?
4 in (10 cm) on each side is the usual request from longarm quilters, because the frame needs something to grip. For quilting on a home machine, 2–3 in on each side is enough. Check with your quilter if someone else is finishing it.
Should backing seams run vertically or horizontally?
Either is fine for the quilt. Pick whichever uses less fabric, which is what the calculator does. Some longarm quilters prefer seams running parallel to their rails, so ask if you're sending it out.
How much binding do I need?
Measure all four sides, add 10 in for joining and corners, and divide by 40 in to get the number of strips. A throw quilt needs 6 strips of 2.5 in: 15 in of fabric, so buy 1/2 yd.
What size batting do I need?
The same size as the backing. Batting by the yard comes 90, 96 or 120 in wide; packs come in crib, twin, full, queen and king sizes. The calculator suggests the smallest pack that fits.
Can I use metres and centimetres?
Yes. Switch the units to metric and the calculator works in centimetres and rounds fabric up to the next 10 cm, the way UK and European shops cut it.
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