How the diagonal backing method works
Instead of sewing two full panels side by side, you take one length of fabric, cut it diagonally from corner to corner, and slide the two halves along the cut until the piece is wide enough. Sew the diagonal seam and trim the ends square. Because you only buy one length, you can save a good amount of fabric on mid-sized quilts.
The formula
Fabric length = backing length × fabric width ÷ (2 × fabric width − backing width)
The backing width here includes about 1" for the seam allowance, measured across the slanted seam. The method only works when the backing is wider than one width of fabric and less than twice as wide.
Worked example: 52" × 82" quilt
With 4" extra on each side the backing is 60" × 90", and the fabric is 42" wide.
- Straight seams: two 90" panels (vertical) or three 60" panels (horizontal): 180" either way, 5 yards.
- Diagonal seam: 90 × 42 ÷ (84 − 61) ≈ 164.6", so buy 4 5/8 yards.
The diagonal seam saves about 15", or 3/8 yard, on this quilt.
When it is worth it
The saving depends on how much wider than the fabric your backing is:
- Up to about 1.5 × the fabric width (about 62" with 42" fabric): diagonal piecing usually saves fabric, as long as the other side is also too long for one width.
- Above 1.5 ×: a straight seam needs the same or less. The calculator tells you when that happens.
- Twice the fabric width or more: the method doesn't work; use straight seams or wide backing.
Step by step
- Cut off the selvages and starch the fabric: the diagonal cut is on the bias and will stretch.
- Fold or mark the fabric and cut from one corner to the opposite corner.
- Slide one half along the cut until the piece is as wide as your backing needs to be.
- Pin closely, sew a 1/2" seam without pulling, and press it open.
- Trim the two pointed ends so the backing is a rectangle of the size shown in the results.