A few useful things to know
How to use this diagonal calculator
John Flynn’s method cuts a rectangle from corner to corner, slides the two triangles along the diagonal, joins them and trims a wider rectangle. The cut is not automatically 45°. We check the supported width range and account for the seam perpendicular to that cut.
- Enter your quilt top and overhang to find the required backing rectangle.
- Use the actual fabric width after removing the selvedges. The supported target width, including a seam reserve, is at most 1½ fabric widths.
- Cut the full rectangle and mark the corner-to-corner line. Keep both triangles facing the same way; slide rather than rotate.
- Align the seam lines, sew with the chosen allowance, press open and check the rectangle before trimming. Handle the long bias edges gently.
A worked example
For a required backing of 51 × 66 inches on fabric with 44 usable inches and ½ inch seam allowances, the seam-aware geometric length is 80.978 inches (rounded up). The cutting plan rounds up to 81 inches. Buy 2¼ yards. A regular horizontal-panel layout needs 102 inches before retail rounding, so the diagonal plan saves 21 inches of cut fabric.
The method behind the result
For target backing B × H and usable fabric width F, ideal seam-free fabric length is L = HF / (2F − B), the same as Flynn’s published equation. With a seam allowance s on each diagonal edge, solve HF = L(2F − B) − 2s√(L² + F²). The extra term accounts for the seam measured perpendicular to the sloping edge.
What this plan assumes
A single non-directional fabric; no pattern matching. Supported when backing width + 2s is no more than 1½ usable fabric widths. If one width already fits, there is no need for a diagonal seam. The calculation models flat, unstretched fabric. A longarm service must accept a diagonal seam before you choose this method.
Common mistakes to avoid
- Assuming a corner-to-corner line on a long rectangle is a 45° line.
- Using the finished quilt size without its backing overhang.
- Rotating a triangle instead of sliding it.
- Stretching the bias seam or assuming a repeating motif will still match.