Measure the quilt top you have
Use the current width and height of your pieced quilt top, measured from edge to edge. Add the overhang your quilting setup needs on each side. Ask your longarm quilter for their requirements before buying fabric; the 4-inch starting value is an example, not a requirement for every quilt.
Choose which way the panels run
Vertical panels run from the top of the quilt to the bottom. Horizontal panels run from side to side. Changing direction can change both the panel count and the length of fabric needed. For a one-way print, keep the fabric in the direction you want it to appear. Allow rotation only if turning the print works for your quilt.
How much fabric does a 60 × 80-inch top need?
With 4 inches of overhang on every edge, the backing needs to cover 68 × 88 inches. Two 40-inch-wide vertical panels joined with ½-inch seams give you 79 inches of joined width. Each panel is 88 inches long, for 176 inches in total. Rounded up in ⅛-yard increments, the purchase amount is 5 yards.
Leave room for print matching
The cut list covers rectangular panels and their seams. It doesn't add fabric to line up a large print, remove a flaw or make a more elaborate pieced back. Work out those extras before ordering. If you already own the fabric, compare its measured length with the raw fabric length in the result, rather than the rounded shop quantity.