Formula
Add the overage to both dimensions. Then piece whichever way uses fewer yards: panels across the width (seams run the length) or panels across the length (seams run the width).
need = quilt + 2 × overage
panels = ceil(span ÷ fabric width)
yards = panels × panel length ÷ 36
buy = yards, rounded up to the next 1/8 yard
This is the pieced-backing method quilt shops publish. The sizes in the table are common finished tops, not a bedding standard.
Worked examples
60 × 80, 4 inch overage, 42 inch fabric. The back must be 68 × 88. Two panels with seams along the length use 176 inches of cloth, 4.889 yards, which cuts as 5 yards.
Same quilt, 108 inch fabric. One panel the short way is 68 inches, 1.889 yards, which cuts as 2 yards. The long way would be more, so the calculator keeps the short way.
Reference
Four inches of overage on every side. “Buy” is already rounded up to the next eighth of a yard. Widths are usable fabric, after you have given up the selvage.
| Top | Cloth | Panels | Exact | Buy |
|---|---|---|---|---|
| Throw 50×65 | 42 in | 2 · seams run the width | 3.222 yd | 3 1/4 yd |
| Throw 50×65 | 108 in | 1 · seams run the width | 1.611 yd | 1 5/8 yd |
| Twin 63×87 | 42 in | 2 · seams run the length | 5.278 yd | 5 3/8 yd |
| Twin 63×87 | 108 in | 1 · seams run the width | 1.972 yd | 2 yd |
| Full 78×87 | 42 in | 3 · seams run the width | 7.167 yd | 7 1/4 yd |
| Full 78×87 | 108 in | 1 · seams run the width | 2.389 yd | 2 1/2 yd |
| Queen 90×108 | 42 in | 3 · seams run the width | 8.167 yd | 8 1/4 yd |
| Queen 90×108 | 108 in | 1 · seams run the length | 3.222 yd | 3 1/4 yd |
| King 108×108 | 42 in | 3 · seams run the length | 9.667 yd | 9 3/4 yd |
| King 108×108 | 108 in | 2 · seams run the length | 6.444 yd | 6 1/2 yd |
Questions
How many yards of backing does a quilt need?
Add the overhang to the width and the length, then piece panels of your fabric width. Panel count is the span divided by the usable width, rounded up. Yardage is panels times panel length, divided by 36, then rounded up to the next eighth of a yard. This page tries both directions and keeps the one that uses less cloth.
Why does 108 inch backing use less than 42 inch backing?
A wide back often covers the quilt in one panel, so you buy the length (plus overhang) instead of two or three lengths of 42 inch cloth. A 60 by 80 quilt with 4 inches of overhang needs a 68 by 88 inch back. One panel of 108 inch fabric is 68/36 yards, which a shop cuts as 2 yards.
How much overhang should backing have?
Four inches past the quilt top on every side is the usual allowance for loading a longarm or for a home-machine sandwich that can shift. That adds 8 inches to the width and 8 inches to the length. You can change it.
Does this include a seam allowance between panels?
The panel count assumes you seam the selvages and use the width you typed as the usable width. If you lose half an inch at each seam, type a slightly narrower usable width. The eighth-yard round-up is the shop cut, not extra seam allowance.
Sources
- Pieced backing: panels = ceil(span ÷ usable width), yards = panels × panel length ÷ 36. The cheaper orientation wins.
- Shop rounding: yardage is sold in eighths. 4.889 yards is cut as 5.