Quilt Backing Calculator

Start with the backing size: quilt top + 2 × the extra per side, in both directions. A 60 × 72-inch throw with 4 inches all round needs a 68 × 80-inch backing. Quilting cotton is narrower than that, so the backing is sewn from full-width panels. Each seam takes a seam allowance from both panels it joins, so two 42-inch panels sewn with ½-inch seams cover 83 inches, not 84.

The number of panels is the smallest whole number that covers the backing: panels = ⌈(backing − 2 × seam) ÷ (usable width − 2 × seam)⌉, or one panel if the backing is no wider than the fabric. Multiply the panels by the length each one is cut to, divide by 36, and round up to the next ⅛ yard. Use the usable width between the selvages. A 44/45-inch quilting cotton usually gives about 42 inches once the selvages are cut off, but measure your own bolt.

With vertical seams the panels hang down the quilt, so every panel is cut to the backing length. With horizontal seams the panels run across it, so every panel is cut to the backing width. Most quilts are longer than they are wide, so when both layouts need the same number of panels, horizontal seams cut shorter panels and use less fabric.

The throw above needs two panels either way. Vertical: 2 × 80 inches = 4 ½ yd. Horizontal: 2 × 68 inches = 3 ⅞ yd, which is ⅝ yd less. It doesn’t always go that way. A 30 × 40-inch baby quilt (38 × 48-inch backing) fits on one width of fabric with vertical seams, which here means no seam at all, for 1 ⅜ yd. Turned sideways it would need two panels and 2 ⅛ yd. The calculator checks both and shows the trimmed-off strip for each, so you can see where the waste goes.

Two things can rule out the cheaper layout. A one-way print turns sideways in horizontal panels, so the directional switch keeps the seams vertical. And some longarm quilters prefer seams to run a particular way on their frame, so ask before you sew the panels together.

On a longarm frame the backing is clamped or pinned to the rails, and the machine needs room to work right up to the quilt’s edge. Quilting also draws the layers in slightly. That is why longarm quilters ask for backing bigger than the top on every side. 4 inches per side (8 inches added to each dimension) is a common request. Some quilters ask for more, especially on large quilts, so check with yours before buying.

If you quilt it yourself, you choose the margin. A few inches per side covers layers that shift while you baste and quilt, and the excess is trimmed off before the binding goes on, so enter whatever margin you plan to use.

Yardage for the backing alone, worked out with the calculator above using 4 inches of extra per side, ½-inch seams and the cheaper seam direction for each fabric. The size names are examples, because quilt sizes vary from pattern to pattern, so use the calculator for your own top.

Wide backing removes the seams for any backing up to its usable width, about 106 inches for a 108-inch bolt. The queen in the chart (90 × 108 inches, 98 × 116-inch backing) takes 3 ¼ yd of wide backing with no seam, against 8 ¼ yd of 44/45-inch cotton sewn from 3 panels. A king with 4 inches all round needs a 116-inch backing, which is wider than 106 inches, so even wide backing needs a seam at that size.

To compare cost, multiply each yardage by that fabric’s price per yard. Wide backing costs more per yard but you buy far fewer yards, and you skip cutting and sewing long seams. Standard-width fabric gives you a much wider choice of prints, and you can piece leftover fabric into the backing.

Cut the batting to the same size as the backing: the quilt top plus the same extra on every side. Then all three layers reach the frame clamps or the basting edge together. The calculator gives this as the batting size. Packaged batting sizes differ from brand to brand, so check the dimensions printed on the package and choose one that is at least that big in both directions.

If the only batting you have is a little too small, two pieces can be butted edge to edge and joined with a wide zigzag stitch or a batting joining tape, without overlapping them. Follow the batting maker’s guidance on how far apart the quilting lines can be. Leftover batting from a big package is useful for small projects.

A 60 × 72-inch top going to a longarm quilter who wants 4 inches per side, backed with 44/45-inch quilting cotton (42 inches usable) and joined with ½-inch seams.

1. Backing: (60 + 8) × (72 + 8) = 68 × 80 inches.
2. Vertical seams: ⌈(68 − 1) ÷ (42 − 1)⌉ = 2 panels × 80 inches = 160 inches = 4.44 yd → 4 ½ yd.
3. Horizontal seams: ⌈(80 − 1) ÷ 41⌉ = 2 panels × 68 inches = 136 inches = 3.78 yd → 3 ⅞ yd.
4. Two panels sewn together cover 83 inches. Horizontal seams need 80 of those, so trim off a 3-inch strip.
5. Batting: 68 × 80 inches.

Buy 3 ⅞ yd, cut it into two 68-inch lengths, and sew them together along a long edge. If the print is one-way, buy 4 ½ yd and cut two 80-inch lengths instead.