Quilt backing guide · formula-review edition
Backing math changes with fabric width and the way panels are joined. Before accepting any calculator result, make each assumption visible.
Use the finished width and length. If a pattern lists unfinished blocks or borders still to be added, resolve those first. A backing estimate cannot correct a top-size assumption that is already wrong.
“Quilt cotton” is not one width. Common options such as 42, 44, 54 and 108 inches can lead to different panel choices, joins and purchase lengths. A calculator should always let you choose or enter the actual usable width.
Quilters may add extra for quilting, squaring, shrinkage or personal practice. The useful result is not the one with a hidden default; it states the allowance and lets you adjust it.
Two totals can look similar while the cut and join layout is very different. A trustworthy result names the number of panels, their orientation and the cut lengths so you can spot a mismatch before purchasing.