A little room around the edges
Enter the quilt top’s width and length. The extra-on-each-side setting adds a border all the way around. For example, 4 inches on each side turns a 60 × 72 inch quilt into a 68 × 80 inch backing. The default is a starting point: use the overhang your quilter asks for.
Measure the fabric you can use
The presets estimate usable widths of 40, 56, and 104 inches after the selvages are removed. Fabric varies. Replace that estimate with the width you actually have, or expect to have after any washing and trimming.
Each seam uses the allowance on both joined edges. This tool calculates enough full-width panels, compares seams in both directions, and chooses the layout that uses less bolt length. It assumes panels can be rotated and that you will trim the joined backing to size. It does not optimize offcuts or piece a backing from scraps.
Before you buy or cut
The purchase quantity rounds up to the next ⅛ yard. Check the increments your shop sells. The centimeter quantity is the same purchase length rounded up to a whole centimeter.
Add any allowance you need for washing, squaring up, or matching a directional print. Those extras are not included. A directional print may also prevent using the shorter seam direction shown here.