Picture the seams on the finished back
Vertical panels run from top to bottom, with the joining seams between them also running vertically. Horizontal panels run across the quilt, with horizontal joining seams. Either arrangement may cover the same rectangle, but it can need a different number of panels and a different total length of fabric.
Compare the same target rectangle
For a 68 × 88-inch backing target, 40-inch usable fabric width and ½-inch seams, a vertical layout needs two panels, each 88 inches long. That is 176 inches of fabric. A horizontal layout needs three panels, each 68 inches long, or 204 inches in total. The panel seams are already allowed for in the joined span.
Check whether turning the fabric is acceptable
A horizontal layout turns the fabric relative to a vertical one. Check directional prints and any grain requirements before allowing that rotation. The backing calculator won't use a horizontal layout with the keep-warp-vertical setting unless you explicitly allow the turn.
Compare fabric on hand before shop rounding
When you already own a piece of fabric, compare its measured length with the raw fabric length in the plan. When you're buying fabric, use the rounded purchase amount. Add any print-matching or flaw-removal allowance separately; neither is included in this panel comparison.