Keep the quilt center the same
A border can be a small design change with a larger effect on the shopping list. Start with one center size and compare both options from there. In this example, the center finishes at 40 × 50 inches. We use ¼-inch seams and attach the top and bottom borders first. Keeping those settings the same makes the comparison easier to follow.
Write down the new quilt sizes
A 3-inch finished border makes the quilt 46 × 56 inches. A 4-inch border makes it 48 × 58 inches. In the border calculator, the 3-inch option needs 3½-inch-wide strips, with pairs 40½ and 56½ inches long. The 4-inch option needs 4½-inch-wide strips, with pairs 40½ and 58½ inches long. Both plans use four border pieces, but their fabric requirements differ.
The binding purchase may stay the same
With 40-inch usable fabric width, 2½-inch binding strips, diagonal joins and a 12-inch tail, both quilt sizes need six binding strips. Those strips use 15 inches of fabric, which rounds up to ½ yard in ⅛-yard increments. The wider border increases the perimeter, but not enough to require a seventh strip in this example.
The backing can cross a shop increment
Now allow 4 inches of backing overhang on every edge, using 40-inch usable width, vertical panels and ½-inch panel seams. The smaller quilt needs two 64-inch panels: 128 inches in total, or 3⅝ yards after purchase rounding. The larger quilt needs two 66-inch panels: 132 inches, rounded to 3¾ yards. The extra raw length is only 4 inches, but the shop quantity rises by ⅛ yard.
Compare the cut list as well as the yardage
Choose the border for the quilt you want to make, then check that the fabric will cover the cuts. The border tool gives continuous strip lengths, not a yardage or piecing plan, so work out any joins needed for long strips separately. Once the borders are sewn, measure the actual top before cutting backing and batting. Use the new finished size when you calculate binding.