What This Calculator Figures Out
Enter your quilt top width and length, pick the fabric width you're buying, and set the overhang, and this tool returns the finished backing size, how many fabric widths to seam, the batting size, and the total yardage. The backing has to end up larger than the top on every side because the quilt sandwich shifts as it's quilted and the longarm frame needs fabric to clamp. The usable width of your fabric decides whether one width does the job or whether you seam two or three together.
A Worked Example
Take a common 60×80" throw or twin top with 4" of overhang on each side. The backing target becomes 68×88". Since 68" is well past the 44" usable width of quilting cotton, one width won't reach — you seam two. Two widths of 88" run about 176" total, close to 5 yards; flipping the seam horizontal to two 68" widths drops it to about 136", roughly 3.8 yards. A single 108" wide backing would cover the 68" width flat and need only the 88" length, about 2.5 yards with no seam.
Why the Fabric Width Drives Everything
Standard quilting cotton is sold at 42–44" but you lose the selvages, so plan on 42" usable. That's why any backing wider than roughly 36" already forces a seam once you add overhang. Piecing isn't a flaw — a well-placed horizontal seam often uses far less fabric than stacking vertical widths, and pressing it open keeps the back flat for quilting. When the seaming math gets expensive, that's the signal to price out 108" wide backing instead.
Quilt Backing Calculator
Standard Quilt Sizes & Backing Yardage
Yardage includes 4-inch overhang per side. Based on 44-inch fabric width.
| Quilt Size | Top Dimensions | Backing Needed | Yards (44") | Yards (108") |
|---|---|---|---|---|
| Baby / Crib | 36" × 52" | 44" × 60" | 3.4 yd | 1.7 yd |
| Throw | 50" × 65" | 58" × 73" | 4.1 yd | 2.1 yd |
| Twin | 68" × 86" | 76" × 94" | 5.3 yd | 2.7 yd |
| Full | 80" × 90" | 88" × 98" | 5.5 yd | 2.8 yd |
| Queen | 86" × 96" | 94" × 104" | 5.8 yd | 3.0 yd |
| King | 104" × 96" | 112" × 104" | 9.3 yd | 3.0 yd |
| Cal King | 104" × 108" | 112" × 116" | 9.7 yd | 3.3 yd |
| Lap Quilt | 40" × 56" | 48" × 64" | 3.6 yd | 1.8 yd |
| Table Runner | 16" × 48" | 24" × 56" | 1.6 yd | 1.6 yd |
How to Use This Calculator
- Measure the Quilt Top: Enter the finished width and length of your quilt top in inches. Measure the actual top, not the bed size, since tops often differ from mattress dimensions.
- Choose the Backing Fabric Width: Pick 42–44" for standard quilting cotton or 108" if you're buying wide backing. This is what decides whether the backing needs seams.
- Set the Overhang per Side: Use 4" per side for most projects, or the number your longarm quilter asked for — some want 4–6" to clamp onto.
- Read the Layout and Yardage: The results show the finished backing size, how many widths to seam and in which direction, the batting size, and total yards to buy.
How It Works
Backing has to finish larger than the quilt top on every side so the longarm frame has fabric to grip and so nothing runs short if the layers shift during quilting. Once the target backing exceeds the usable width of your fabric — about 42–44 inches for standard quilting cotton — the panel has to be pieced from two or three widths seamed together.
The basic rule:
- Add 4 inches of overhang on each side, so the backing measures 8 inches wider and 8 inches longer than the top
- If the target backing width is more than the usable fabric width, you seam two or three widths together to reach it
- On 42–44" cotton, any quilt wider than roughly 36" needs at least one seam; a 108" wide back covers most beds seamless
The calculator compares your target backing size against the fabric width, decides how many widths it takes, runs the seam either vertically or horizontally to use less fabric, and totals the yardage for you.
Tips & Considerations
- Measure the actual finished top rather than the bed size — pieced tops shrink a bit at every seam and often come out smaller than planned.
- When two widths give you a lot of leftover, try flipping the seam direction; a horizontal seam frequently uses a yard less than stacking vertical panels.
- Trim the selvages off before seaming — leaving them in creates a tight, wavy seam that fights the longarm.
- Buy an extra quarter-yard beyond the calculated number for squaring up the backing and absorbing shrinkage.
- Place the backing seam off-center so it doesn't line up under a densely quilted block, where the extra bulk shows most.
Frequently Asked Questions
How much bigger than the quilt top should the backing be?
Aim for 4 inches of extra backing on each side, which makes the backing 8 inches wider and 8 inches longer than the top. So a 60×80" top wants backing around 68×88". Many longarm quilters ask for a full 4–6 inches per side because their clamps and leaders eat into the edges — always confirm the number your quilter wants before you cut.
When do I have to piece the backing instead of using one width?
You piece as soon as your target backing dimension is wider than the usable fabric. Standard quilting cotton gives about 42–44" once you trim the selvages, so a backing that needs to be 68" wide can't come from a single width. Two widths seamed together give you roughly 84–88" of usable width, which covers throws and twins; three widths handle queen and king backs. A 108" wide backing avoids seams on almost everything.
Should the seam run horizontally or vertically?
Run the seam whichever direction uses less fabric. For a 68×88" backing from 44" cotton, two vertical widths of 88" each is about 176" (≈5 yards), while two horizontal widths of 68" each is about 136" (≈3.8 yards) — the horizontal layout wins here. The calculator tries both orientations and reports the cheaper one. Press the seam open and keep it off-center so it doesn't land under a heavily quilted block.
What is wide backing fabric and when is it worth it?
Wide backing is quilting cotton woven at 108" (3 yards) wide specifically so you can back most quilts with a single seamless panel. A queen backing of 94×104" needs just about 3 yards of 108" fabric flat, versus three seamed widths of 44" cotton totaling roughly 8.7 yards. It costs more per yard but you skip the seaming, and there's no seam ridge for the longarm to bump over.
Do I calculate batting the same way as backing?
Yes — batting matches the backing, not the top. Cut or buy batting at least 4 inches bigger per side, so a 60×80" top takes batting of at least 68×88". Batting comes in standard package sizes (crib 45×60", twin 72×90", full 81×96", queen 90×108", king 120×120"), so round up to the next size that covers your number rather than buying exactly to fit.
How much backing does a king quilt take?
A king top runs about 104×96". With 4" overhang per side that's 112×104" of backing. From 44" cotton you need three widths of 112" each — roughly 9.3 yards — sewn with two seams. From 108" wide backing you need about 104" of length turned sideways, close to 3 yards with no seams at all. The wide backing is usually the better buy at king size.
Should I pre-wash backing fabric?
If you'll wash the finished quilt and want it to stay flat, pre-wash the backing so it shrinks before it's quilted in — most quilting cotton shrinks 3–5%, and unshrunk backing against a washed top can pucker. If you're after the crinkled, antique look, skip pre-washing and let everything shrink together in the first wash.