How to use the backing calculator
- Pick a quilt size or type the measurements of your finished top. Measure through the middle in both directions, since the edges can stretch.
- Set the extra backing per side. Four inches is the default; a longarm quilter may ask for more.
- Tap a fabric width or type your own. Quilting cotton is usually 42 or 44 inches, and wide backing is 108 or 118.
- Leave the selvage trim at 2 inches and the seam allowance at ½ inch unless you work differently.
- Tick directional print if the fabric has a one-way design.
How backing math works
The backing needs to be bigger than the top, because the layers shift a little while you quilt, so the calculator adds the extra to every side.
Then it works out how many widths of fabric it takes to cover one direction. Selvages come off first, and every seam eats two seam allowances, so three panels of 40 usable inches with ½ inch seams only cover 118 inches, not 120. The number of panels times the length of each panel is the fabric to buy.
Panels can run down the length of the quilt (vertical seams) or across it (horizontal seams). The two layouts often need different amounts. The calculator works out both and recommends the smaller one, unless the print has to stay upright.
Formulas
- Backing size = (width + 2 × extra) × (length + 2 × extra).
- Usable width = fabric width − selvage trim.
- Panels = (backing span − 2 × seam) ÷ (usable width − 2 × seam), rounded up (1 if it already fits).
- Joined span = panels × usable width − (panels − 1) × 2 × seam.
- Yards = panels × panel length ÷ 36, rounded up to ⅛ yd.
Worked example: a queen quilt
The top is 90 by 95 inches. With 4 inches on every side, the backing has to be 98 by 103 inches. Fabric 42 inches wide gives 40 usable inches.
For vertical seams the panels sit side by side across 98 inches: (98 − 1) ÷ 39 = 2.49, so 3 panels, each 103 inches long. That is 309 inches, or 8⅝ yards. For horizontal seams the panels stack down 103 inches: (103 − 1) ÷ 39 = 2.62, so 3 panels of 98 inches, which is 294 inches or 8¼ yards.
Horizontal seams save ⅜ yard here. Sewn together, the three panels measure 118 inches, so trim 15 inches off after joining.
Backing yardage for standard quilt sizes
With 4 inches extra per side, 2 inches of selvage trim and ½ inch seams.
| Size | Backing | 42" fabric | Layout | 108" wide |
|---|---|---|---|---|
| Crib 36" × 52" | 44" × 60" | 2½ yd | 2 panels, 1 horizontal seam | 1¼ yd |
| Throw 50" × 65" | 58" × 73" | 3¼ yd | 2 panels, 1 horizontal seam | 1⅝ yd |
| Twin 68" × 86" | 76" × 94" | 5¼ yd | 2 panels, 1 vertical seam | 2⅛ yd |
| Full 84" × 90" | 92" × 98" | 7¾ yd | 3 panels, 2 horizontal seams | 2⅝ yd |
| Queen 90" × 95" | 98" × 103" | 8¼ yd | 3 panels, 2 horizontal seams | 2¾ yd |
| King 108" × 95" | 116" × 103" | 8⅝ yd | 3 panels, 2 vertical seams | 3¼ yd |
When you need one, two or three widths
Backing yardage jumps in steps, because each extra width of fabric adds a whole panel. This table shows how far sewn widths reach with selvages trimmed and ½ inch seams, and the widest quilt top each one backs with 4 inches extra per side.
| Widths sewn | 42" fabric reaches | Widest top, 42" | 44" fabric reaches | Widest top, 44" |
|---|---|---|---|---|
| 1 | 40" | 32" | 42" | 34" |
| 2 | 79" | 71" | 83" | 75" |
| 3 | 118" | 110" | 124" | 116" |
The Missouri Star backing guide points out that full and queen backings nearly double in yardage because they cross into a third width. A small change in overage can do the same. A twin top is 68 inches wide, so with 4 inches per side the backing fits in two widths. With 6 inches per side it needs 80 inches, one more than two widths cover, and the yardage goes from 5¼ to 6¾ yards.
Is wide backing cheaper?
Wide backing costs more per yard, but you buy far fewer yards. Divide the 42" yardage by the 108" yardage and you get a break-even price. Wide backing is the cheaper choice whenever its price per yard is less than that many times the price of the 42" fabric.
| Size | 42" fabric | 108" wide | Wide is cheaper below | 42" with 6" per side |
|---|---|---|---|---|
| Crib 36" × 52" | 2½ yd | 1¼ yd | 2.0 × the 42" price | 2¾ yd |
| Throw 50" × 65" | 3¼ yd | 1⅝ yd | 2.0 × the 42" price | 3½ yd |
| Twin 68" × 86" | 5¼ yd | 2⅛ yd | 2.5 × the 42" price | 6¾ yd |
| Full 84" × 90" | 7¾ yd | 2⅝ yd | 3.0 × the 42" price | 8 yd |
| Queen 90" × 95" | 8¼ yd | 2¾ yd | 3.0 × the 42" price | 8½ yd |
| King 108" × 95" | 8⅝ yd | 3¼ yd | 2.7 × the 42" price | 10 yd |
For a queen, 108" backing wins if it costs less than 3.0 times as much per yard as the cotton you would piece. Missouri Star says that from twin size up, wide backing is usually cheaper. The Stitchin' Heaven yardage guide says it often costs about the same and can be cheaper for queen and king quilts. The trade-off is choice, since fewer prints come in 108 inch widths.
Which way the seams should run
- Longarm quilting. Designed to Quilt says longarm quilters usually prefer the backing seam to run parallel to the rollers on their frame. A seam across the rollers builds up bulk in one spot. Which direction that is depends on how they load your quilt, so ask before you cut.
- Bed quilts. Missouri Star suggests lengthwise seams for larger quilts, since the seam then runs along the mattress. For quilts under about 60 inches wide, crosswise seams often save fabric. In the table above, the twin and king are the standard sizes where vertical seams need less fabric.
- A centered seam. Two full widths put a seam down the middle. Missouri Star's alternative is to cut one width in half lengthwise and sew a half to each side of a full width. That moves the seams toward the edges and uses the same yardage.
- Matching a large print. To line up the pattern across a seam, allow one extra pattern repeat per seam. The queen backing above has 2 seams, so a 24 inch repeat adds 48 inches, about 1⅜ yards. This calculator does not add repeats, so add them yourself.
For step-by-step piecing, including pressing and trimming the panels, see the guide to how to piece a quilt back.
Tips for backing
- Cut the selvages off before you sew panels together. They have a tighter weave than the rest of the fabric and can pucker a seam.
- Press backing seams open so the quilt lies flat and the seam is easier to quilt over.
- Buy all the panels from one bolt so the color matches across each seam.
- If you prewash the quilt top fabrics, prewash the backing too so both shrink the same amount. Missouri Star suggests buying about 5 percent extra backing to cover shrinkage.
