How to calculate quilt backing
Backing size = quilt top + 2 × extra (on width and on length)
Panels = (backing width − 1") ÷ (fabric width − 1"), rounded up
Fabric = panels × backing length, rounded up to the next 1/8 yard
Each seam uses about 1" of width (a 1/2" seam allowance on each panel), which is why 1" is taken off. For horizontal seams, swap width and length: the panels run across the quilt and you need panels × backing width.
Worked example: 60" × 72" quilt
With 4" extra on each side the backing must be 68" × 80". Using 42" usable fabric:
- Vertical seam: (68 − 1) ÷ 41 = 1.6, so 2 panels of 80" = 160", or 4 1/2 yards.
- Horizontal seam: (80 − 1) ÷ 41 = 1.9, so 2 panels of 68" = 136", or 3 7/8 yards.
- 108" wide backing: 68" fits within the width, so buy 68", or 2 yards, with no seam.
Turning the seam saves 5/8 yard here. That's why the calculator always checks both directions.
Which layout should you choose?
- Least fabric: the calculator marks it. For many throw and lap quilts it's a horizontal seam or a diagonal seam.
- Wide backing is usually the simplest choice for queen and king quilts.
- Directional prints need the panels to run the right way, which can mean more fabric.
Sewing the backing
- Cut off the selvages: they are tighter woven and can pucker.
- Sew the panels with a 1/2" seam and press the seam open to reduce bulk.
- Square up the backing and check it is at least the size shown above before sending it to your quilter.