Enter your quilt top size and the backing fabric you plan to use. The calculator prices seams that run lengthwise, crosswise, or diagonally, then gives you the yardage, the cuts, and the piecing plan for whichever wastes the least fabric, two widths or a large top backed in thirds.
Your Quilt and Fabric
Units
Backing fabric width after the selvages come off
Fine-tune: seam allowance and fabric price
You Need
8 ¼yards
of 44/45″ cotton · seams run crosswise
- Backing size needed98 in × 108 in
- Cut3 widths of fabric × 98 in long
- Pieced backing width124 in
- Leftover strip16 in × 98 in strip
- Turning the seams saves¾ yd
- Batting that fitsKing pack (120 × 120 in)
- Estimated cost:
tinted: backing · dashed: seams · outlined: your quilt top
The Formulas This Calculator Uses
Backing must be bigger than the quilt top on every side. With your overhang o:
backing width = quilt width + 2·o · backing length = quilt length + 2·oLengthwise or Crosswise Seams
When the backing is wider than the fabric, you piece widths of fabric together. Each seam takes up twice the seam allowance s, so with usable fabric width F:
widths = ⌈(backing width − 2s) ÷ (F − 2s)⌉ · fabric to buy = widths × backing lengthCrosswise seams use the same formula with width and length swapped. Whichever direction takes fewer total inches wins, and it flips as quilts change shape, so the calculator prices both and shows you the difference instead of silently picking one.
The Diagonal Seam (John Flynn Method)
For backings up to about one and a half times the fabric width, one length of fabric cut corner-to-corner and slid along the diagonal makes a wider back from less yardage than two straight panels. Flynn’s published formula, with W = backing width + 1 in for the seam:
fabric length = backing length + backing length × (W − F) ÷ (2F − W)The calculator offers this layout automatically whenever your numbers qualify, and tells you exactly how much it saves. The one trade-off: the seam runs visibly on the diagonal, so it suits busy prints better than solids.
Worked Example
A 50 × 65 in throw quilt, 4 in overhang, 42 in usable quilting cotton, ½ in seams:
- Backing needed: 50 + 8 = 58 in wide, 65 + 8 = 73 in long.
- Lengthwise seams: ⌈(58 − 1) ÷ 41⌉ = 2 widths × 73 in = 146 in → 4⅛ yd.
- Crosswise seams: ⌈(73 − 1) ÷ 41⌉ = 2 widths × 58 in = 116 in → 3¼ yd, the least fabric here.
- Diagonal: W = 59 is under 1.5 × 42 = 63, so it qualifies. 73 + 73 × (59 − 42) ÷ (84 − 59) = 122.6 in → 3½ yd.
- So: an all-over print takes crosswise seams and 3¼ yd. A directional print can’t take crosswise seams, and there the diagonal saves ⅝ yd over lengthwise widths. That’s why the calculator prices all three instead of picking one for you silently.
Backing Yardage Chart
Computed by the calculator above: 4 in overhang, ½ in seams, cheapest straight seams. Diagonal savings shown where the quilt qualifies.
| Quilt | Backing size | 42″ cotton | Layout | Diagonal | 108″ wide back |
|---|---|---|---|---|---|
| Crib 36×52 | 44 × 60 in | 2 ½ yd · 2 widths | crosswise | 1 ⅞ yd: saves ⅝ | 1 ¼ yd · no seam |
| Throw 50×65 | 58 × 73 in | 3 ¼ yd · 2 widths | crosswise | , | 1 ⅝ yd · no seam |
| Twin 70×90 | 78 × 98 in | 5 ½ yd · 2 widths | lengthwise | , | 2 ¼ yd · no seam |
| Full 84×90 | 92 × 98 in | 7 ¾ yd · 3 widths | crosswise | , | 2 ⅝ yd · no seam |
| Queen 90×100 | 98 × 108 in | 8 ¼ yd · 3 widths | crosswise | , | 3 yd · no seam |
| King 108×108 | 116 × 116 in | 9 ¾ yd · 3 widths | lengthwise | , | 3 ¾ yd · no seam |
How to Measure Your Inputs
- Quilt top: lay it flat and measure through the middle, not along the edges: edges stretch. Measure width and length to the nearest quarter inch.
- Usable fabric width: measure selvage to selvage, then subtract the selvages and any printed margin. Quilting cotton sold as 44/45 in usually gives 40–42 usable inches.
- Overhang: ask your longarm quilter what they require before you buy: 4 in per side is a common minimum, 6 in is safer. Quilting it yourself on a domestic machine, 2–3 in per side is enough.
Assumptions and Defaults
- Yardage rounds up to the next ⅛ yard everywhere: calculator, chart, and example agree. In metric, results round up to the next 0.1 m.
- Seams are ½ in, pressed open, unless you change them. Panels are joined on the straight grain with selvages removed.
- Shrinkage is added only when you tick “I prewash”: 5% on the fabric you buy, since cotton shrinks mostly along its length.
- A directional print rules out crosswise seams, because those widths would turn your print sideways. The diagonal keeps the print upright, but its seam shows on the bias.
- Cut lengths are exact; many quilters add 2–4 in of squaring-up slack per cut on top of these numbers.
Backing Questions, Answered
How much bigger should backing be than the quilt top?
Quilt backing should be at least four inches bigger than the top on every side for longarm quilting, so add 8 inches to both the width and the length. Many longarmers ask for 6 inches per side on larger quilts, whereas 2 to 3 inches per side is plenty when you quilt it yourself, and when in doubt, ask whoever is quilting it before you buy fabric.
Should backing seams run lengthwise or crosswise?
Backing seams should run in whichever direction uses less fabric, and that changes with the quilt’s shape, so price both, as this calculator does. Two exceptions override the math: a directional print must keep its seams lengthwise, and on a bed quilt many quilters prefer a lengthwise seam because it lies along the mattress and wears better through washing. Some longarmers also ask for crosswise seams because they load with less bulk on the frame, so it is worth checking with yours.
When is the diagonal (John Flynn) backing worth it?
The diagonal (John Flynn) backing is worth it when your backing width is more than the fabric width but no more than about 1.5 times it, which is typical for crib, throw, and narrow twin quilts on 42 in cotton. In that range it uses less fabric than vertical panels, and it saves money whenever a directional print rules out horizontal seams; skip it for solids and high-contrast fabrics, where a bias seam would show.
How many yards of 108-inch wide back do I need?
For almost every quilt up to king size, 108 in wide backing takes the quilt’s length plus 8 in of overhang, divided by 36, which usually comes to 3 to 3¼ yards with no seams to sew. Choose 108 in wide back in the fabric menu above, and the calculator works out the exact figure for your quilt.
Can I use a flat sheet as quilt backing?
Yes, you can use a flat sheet as quilt backing, and the sizes often work, since a full flat sheet backs most throws. Choose 100% cotton with a looser weave, percale rather than high-thread-count sateen, because dense sheets are hard to needle and can cause skipped stitches on longarm machines, then prewash it, cut off the hems, and enter its width as a custom width in this calculator.
What seam allowance should I use for backing seams?
The usual seam allowance for backing seams is half an inch, pressed open, which spreads the bulk so the seam doesn’t leave a ridge under dense quilting. A quarter-inch seam works too, as long as you press it open rather than to one side.
Do I need to prewash backing fabric?
Whether you need to prewash backing fabric depends on the top: if the top’s fabrics were prewashed, prewash the backing, and if not, leave it unwashed. Mixing washed and unwashed cotton is what causes uneven puckering after the first wash, and if you do prewash, buy about 5% extra to cover shrinkage.
Which way does the leftover strip run, and what is it good for?
The leftover strip is the surplus width of the last panel, and it runs the full length of the backing, with its size shown by the calculator. Quilters commonly use it for the hanging sleeve, a matching binding, or a pieced label strip, so it rarely goes to waste.
Why does my fabric store’s answer differ from this one?
A fabric store’s answer usually differs from this one for three reasons: a different overhang assumption (2, 4, or 6 inches per side), a different usable-width figure for the same bolt, or rounding to the half yard instead of the eighth. Set those three inputs above to match theirs, and the two numbers will agree.