Most tools give you one yardage number. The real decision is which layout wastes least — and it flips with the quilt's proportions. This page compares three layouts side by side, so you can see the trade-off between fabric and seams before you cut.
Backing needed: width = top width + 2 × overage, length = top length + 2 × overage. With the defaults above that is 68 × 88 in — notice the backing is bigger than the 60 × 80 in top, which is why backing eats more fabric than the top itself.
Vertical seams: panels run along the length, side by side across the width. Panels = ceil(backing width ÷ effective panel width). Horizontal seams: the same arithmetic transposed — panels = ceil(backing length ÷ effective panel width), panels stacked along the length. Wide-back: one wide piece, panels = ceil(backing width ÷ wide-back effective width); with a 68 in wide backing and 108 in fabric that is a single piece with no seams.
Effective panel width = usable width − seam allowance. That is a conservative choice: it counts one seam allowance against each panel, so the panel count never comes out short. Total fabric length = panels × the other dimension; yardage = total length ÷ 36. Seam count = panels − 1. If a layout's backing width exceeds the wide-back width, it is pieced the same way as the vertical layout.
Estimates only, for planning a cut. Measure your actual fabric's usable width after trimming selvedges — printed widths are often wider than what you can really use — and check your quilter's overage requirement before ordering. This page does not sell fabric and does not recommend a purchase; the cheapest layout in fabric may not be the one you prefer if you want fewer seams.
A long, narrow quilt usually wins with vertical seams; a wide, short quilt can win with horizontal seams. Wide-back almost always wins on yardage when it fits in one piece, but it costs more per yard and comes in fewer prints — so compare the price per yard too, not just the yardage.