Calculator
Quilt Backing Calculator
Quilt backing calculator: enter your quilt size to get backing fabric in 42 in usable cotton or 108 in and 120 in wide-backs, both seam layouts compared.
Backing fabric to buy
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Drawn to scale
The outer rectangle is the backing; the dashed rectangle is your quilt top. Panels are hatched in different directions and seams are thick dashed lines. Grid squares are 6 in. The cut list below says the same in words.
Show the math
Cut list
Say your quilt top is 60 x 72 inches. With 4 in extra per side and 42 in usable cotton (our assumption; see Usable width), this quilt backing calculator gives 3 7/8 yd of backing (the quilt’s bottom layer), with one side-to-side seam. The rule: add the extra, count the fabric widths (panels) needed (each seam costs 1 in), multiply by the panel length, divide by 36, and round up to the next 1/8 yd. Show the math, above, runs it on your numbers. New quilt? Start at the quilt size chart.
How much extra backing per side
The extra “covers the fabric that draws up during quilting and gives a longarm machine enough to clamp onto,” says Missouri Star Quilt Co. (MSQC). A longarm is a frame-mounted quilting machine, or its operator. This calculator defaults to 4 in per side, like MSQC, which adds that longarm quilters often ask for 4–6 in; ask yours. Fat Quarter Shop recommends at least 10 in larger each way (5 in per side) and notes “your personal longarm quilter might recommend a different measurement.” Neither Missouri Star nor Fat Quarter Shop gives a home-quilting figure; fabric still draws up there, so lower the extra with care.
Usable width of cotton and wide-backs
Usable width is the WOF (width of fabric) left after trimming the selvages, the tightly woven factory edges. Shops and makers list quilting cotton at 42 to 44 in (MSQC; Robert Kaufman lists Kona Cotton at 44 in), and trimming and squaring up take some of that. MSQC figures 40 in usable. We assume 42 in; yours may differ, so trim one width, measure it, and enter that. OLFA, the rotary-cutter maker, says to cut away from yourself and close the blade after every cut.
Wide-backs are woven wide so many quilts need no seam. Hancock’s of Paducah lists its wide backings at 104 to 120 in, and Quilted Joy lists a Moda 120 in muslin at 112–120 in, so a 120 in back can measure under our 116. We assume 104 in usable for a 108 in back and 116 in for a 120 in back, allowing for selvages and shrinkage; measure yours.
Why each seam costs 1 inch of width
The seam allowance comes off both panel edges at each seam, so 1/2 in uses 1 in of width. On 42 in cotton, each panel after the first adds 41 inches. Fat Quarter Shop and MSQC both say to sew backings with a 1/2 in seam, pressed open. Broadcloth Studio says you can also use 1/4 in, something to experiment with. When you press the seam open, Hamilton Beach’s iron manual warns: “Burns can occur from touching hot metal parts, hot water, or steam.”
Seams up and down or side to side
With seams up and down, panels run the quilt’s length: you need the panel count times the backing length. Side to side, they run across: the panel count times the backing width. A wide-back too narrow one way may fit turned sideways. The tool tries both and picks the one using less fabric (on a tie, fewer seams). A directional print, one with a clear up and down, “overrides the fabric math,” MSQC says, so set Seams run yourself.
Rounding up to the next 1/8 yard
The tool divides by 36 and always rounds up to the next 1/8 yd, because a short backing can’t be stretched. Backside Fabrics sells by the 1/4 yd, and MSQC rounds up to the next 1/4 yd; if your shop cuts in quarters, do the same.
Two worked examples
Example 1: 60 x 72 on 42 in usable cotton (open it). The backing is 68 x 80 in. Up and down, 68 in needs 2 panels of 80 in: 160 in = 4.44 yd → 4 1/2 yd. Side to side, two panels cover 83 in (42 + 41), so 2 x 68 in = 136 in = 3.78 yd → 3 7/8 yd. Two inputs can flip this: 4 3/4 yd at 6 in extra per side, 4 1/2 yd at 40 in usable. In both flipped cases, side to side needs a third panel, so up and down wins.
Example 2: 90 x 108 on a 108 in wide-back at 104 in usable (open it). The backing is 98 x 116 in. Its 98 in width fits within 104 inches: one 116 in piece, no seams, 3.22 yd → 3 1/4 yd.
Next: binding
Work out the binding (the fabric strip that wraps the quilt’s edges) before you shop, so you buy both cuts at once. The “Use this quilt size in the binding calculator” button, just below the inputs, carries your size across. Or open the binding calculator and enter them.
FAQs
How do I calculate how much backing I need for a quilt?
Follow the five-step rule at the top of this page. The two worked examples show it with real numbers.
How much 108 backing do I need for a quilt?
If either backing dimension fits within your trimmed usable width (about 104 in), buy the other dimension, divided by 36 and rounded up (Example 2). A 100 x 100 in quilt needs a backing 108 x 108 in. That’s two panels (6 yd) of a 108 in back at our 104 in usable assumption, or one 3 yd piece of a 120 in back if yours measures at least 108 in after trimming.
How much bigger than the quilt top should the backing be?
Four inches per side is the common default, but ask your longarm quilter (more on extra).
What seam allowance should I use for quilt backing?
A 1/2 in seam, pressed open (Fat Quarter Shop, MSQC). Trim the selvages first, cutting away from yourself, and don’t leave the iron unattended while it’s plugged in (Hamilton Beach) (why each seam costs 1 in).
What is the standard width of quilt backing fabric?
Cotton is listed at about 42–44 in, and wide-backs are most often sold as 108 in or 120 in. We assume 42, 104 and 116 in usable, and you can change each (usable width).