Start with the backing target
Add the selected overhang twice to each quilt dimension. A 60 × 80-inch top with 4 inches extra on every side needs a backing at least 68 × 88 inches. The overhang default is only a planning assumption: your quilter or quilting method may require a different amount.
Compare both directions
With 42-inch usable fabric and a ½-inch seam allowance on each joined edge, two panels join to 83 inches. Vertical panels cut 88 inches long cover the 68-inch target width, requiring 176 inches, or 4.89 yards. Round up to 5 yards when buying quarter-yard increments.
Why the horizontal option can use more
Turning the same fabric around means the joined panel widths must cover the 88-inch target length. Two panels reach only 83 inches, so three 68-inch cuts are needed. That is 204 inches, or 5.67 yards, rounded to 5.75. Directional prints may still make one orientation preferable.
Allowances beyond this calculation
The tool uses full-width rectangular panels of equal cut length. It does not optimize offcuts or account for shrinkage, pattern matching, damaged fabric, straightening or a quilter’s preferred seam placement. Add those needs separately before purchasing or cutting.
Questions, answered
What does seam allowance per edge mean?
A join consumes one allowance from each of the two fabric edges. At ½ inch per edge, each seam reduces the combined width by 1 inch.
Can I use wide backing fabric?
Yes. Enter its actual usable width. If one panel covers the target across the roll, no joining allowance is consumed.