Measure the top and every fabric piece
Write down the edge-to-edge size of the completed quilt top, then the usable width and length of each possible backing fabric. Exclude the selvages and any damage. If you've washed the fabric, measure it now. The original purchase quantity is less useful than the clean rectangle still available on your cutting table.
Work out the target rectangle
For a 60 × 80-inch top with 4 inches of overhang on each edge, the backing needs to cover 68 × 88 inches. Confirm the overhang with your quilting setup or longarm quilter. Don't subtract it to make a fabric piece appear to fit; that extra room has a job to do during basting and quilting.
Check whether the panels fit the pieces you have
With 40-inch usable fabric width and ½-inch seams, two vertical panels give a joined width of 79 inches. Each must be 88 inches long, so the raw fabric requirement is 176 inches. A continuous piece 4⅞ yards long is 175½ inches and falls short, even though it looks close on a yardage list. Two separate pieces can work if each has the required width and at least 88 inches of usable length.
Try another direction only if the print allows it
For the same target, a horizontal layout needs three 68-inch panels, or 204 inches of fabric. That's more fabric in this example, but comparing directions can still be useful with other quilt proportions. Keep a one-way print facing as intended. The calculator doesn't check motifs or find a custom arrangement of differently sized remnants.
Separate a stash check from a shopping quantity
For fabric you already own, compare panel sizes and raw cutting length. For a new purchase, use the rounded fabric-to-buy amount and add any print-matching allowance you need. If the pieces don't fit either simple layout, consider a deliberately pieced back with its own measurements instead of forcing the panel calculator to represent it.