Quilt Backing Calculator

Enter the finished size of your quilt top and the extra your quilter needs. The calculator finds the backing size, compares vertical and horizontal seams to use the least fabric, and shows yardage for standard 42-inch quilting cotton or 108-inch wide backing.

How much bigger should the backing be?

Backing always needs to be larger than the quilt top, because fabric draws up as it is quilted and because frames and longarm machines need extra fabric to hold on to. Most longarm quilters ask for 4 inches of extra backing on every side — 8 inches added to both the width and the length — and some ask for 6 inches for large quilts or dense quilting. If you quilt on a domestic machine, 2 to 3 inches per side is usually enough. Always check with your quilter before cutting, because a backing that is too small is a problem that cannot be fixed on the frame.

How the calculator chooses seams

Standard quilting cotton is about 42 to 44 inches wide, and after trimming selvages you can rely on roughly 40 to 42 inches. Most backings are wider than that, so you sew two or three widths of fabric together. The seams can run the length of the quilt (vertical) or across it (horizontal), and the choice can change the yardage considerably.

1. Backing width = quilt width + 2 × extra; backing length = quilt length + 2 × extra.
2. Vertical seams: number of widths = backing width ÷ usable fabric width, rounded up (allowing for seam allowances). Each width is cut to the backing length.
3. Horizontal seams: number of widths = backing length ÷ usable width, rounded up. Each width is cut to the backing width.
4. Yardage = number of widths × cut length ÷ 36. The calculator shows both and recommends the smaller.

Worked example

A 60 × 72 inch lap quilt with 4 inches of extra on every side needs a 68 × 80 inch backing. With 40 inches of usable width, vertical seams need 2 widths, each 80 inches long — 160 inches, or 4.44 yards, so 4½ yards. Horizontal seams need 3 widths of 68 inches — 204 inches, or 5⅝ yards. Vertical seams save more than a yard. A 108-inch wide backing would need only 80 inches of length: 2¼ yards.

Standard quilt sizes and typical backing yardage

Wide backing fabric

Wide backs are sold in widths of about 90, 108 and 118 inches. They avoid seams entirely for most quilts up to queen size and are often cheaper per square inch than standard cotton. The calculator assumes the wide fabric's usable width (106 inches for a 108-inch bolt after trimming) and, if the backing fits across it, orders enough length for the other dimension. For king quilts larger than the wide fabric, you still need a seam or a narrow extra strip.

Piecing a backing neatly

• Remove selvages before sewing — they shrink differently and can pucker.
• Use a ½-inch seam allowance and press seams open to reduce bulk on the frame.
• Centre a single seam, or place two seams symmetrically, so the back looks planned.
• Use leftover blocks or a strip of contrasting fabric as a design element in the backing to stretch yardage.
• Square the backing: the corners should be 90 degrees so it loads straight on a longarm.
• Mark the top of the backing with a pin if the print has a direction.

Making up a shortfall

If the fabric you love is a little short, add a pieced strip: a column of leftover blocks or a band of a coordinating fabric sewn between two panels can add 10 inches or more of width. Adding a strip in the middle of the backing looks intentional; adding it at one edge often looks like a mistake. Remember that the extra 4 inches on each side will be trimmed away after quilting, so any piecing within that margin disappears.

Batting size

Batting should be slightly smaller than the backing but larger than the top — typically 2 to 3 inches larger than the top on each side for longarm quilting. Many quilters buy batting cut to the same size as the backing and trim it on the frame. Packaged batting comes in crib, twin, full, queen and king sizes; check the dimensions on the package against your backing size rather than relying on the name.

Pre-washing and shrinkage

Whether to pre-wash is a matter of preference, but it affects backing yardage. Quilting cotton can shrink roughly 2 to 5 percent in length and width in the first wash, so if you pre-wash, measure the usable width afterwards and enter that figure. If you do not pre-wash, the finished quilt will shrink slightly after its first wash along with the batting, which many quilters like for the soft, crinkled look. Either way, buy a little extra when the calculation lands close to a yardage boundary.