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How it works Total binding length equals the full quilt perimeter plus one total joining and overlap allowance. The default 12-inch allowance is added once. It does not mean 6 inches on each end.
Worked example A 60 × 80-inch quilt has a 280-inch perimeter. Add 12 inches once for 292 inches of binding. With 2½-inch strips and 40 inches of usable fabric, cut 8 strips and purchase 5/8 yard.
Straight-grain binding Cut the calculated number of width-of-fabric strips. Join strip ends at right angles with diagonal seams. Trim each seam allowance, press it open, and press the completed binding in half. Leave loose working tails while attaching; use the selected strip width as the final cutting overlap. Watch How to Make Straight-Grain Quilt Binding
Continuous bias The calculator shows the final practical measurement for one fabric square. For the 60 × 80 example, cut one 29½ × 29½-inch fabric square. You need one fabric square—not two squares.
Cut that one square diagonally from corner to corner. This creates two triangles from the same fabric square. Reposition and sew those two triangles into a parallelogram. Mark parallel lines at the selected strip width, offset and sew the tube, then cut the continuous spiral. Check that the finished strip meets the required length before attaching it. Open the Continuous Bias Binding Guide
Before cutting Measure the finished quilt top. Confirm the selected strip width. Check that the bias square fits the usable fabric width. Calculate, then choose Print Binding Instructions for the one-page illustrated sheet. Watch How to Add Straight Borders to a Quilt