Plan backing panels without losing seam allowance

The backing starts larger than the quilt top

Enter the quilt top's width and length, then choose the extra backing needed on each side. A 4-inch margin adds 8 inches to the width and 8 inches to the length. For a 60 × 72-inch top, that means a backing target of 68 × 80 inches.

Quilt & Piece starts with a 4-inch margin per side. Your longarm quilter may request something different, so confirm their requirement before ordering fabric. The margin field is editable because a calculator cannot know how your quilt will be loaded or finished.

Seams make the joined backing narrower

Two fabric panels do not contribute their full widths after they are sewn together. Each seam consumes the allowance from both panels. With a 1/2-inch seam allowance, one join removes 1 inch from the combined span.

For n equal-width panels, the joined span is n × usable fabric width − (n − 1) × 2 × seam allowance. The calculator checks this span before deciding how many panels you need. Enter usable width after selvage removal, rather than counting the selvages as part of the backing.

Compare layouts with the same assumptions

For the 68 × 80-inch backing target, 42-inch usable fabric and 1/2-inch seams allow two panels to cover 83 inches. In the vertical-seam layout, cut two panels 80 inches long. That uses 160 inches along the bolt: about 4.444 yards, rounded up to 4 1/2 yards.

In the horizontal-seam layout, cut two panels 68 inches long. That uses 136 inches, or about 3.778 yards, rounded up to 3 7/8 yards. Both layouts cover the target in this example. The automatic option selects the smaller raw yardage, then rounds the shopping amount upward to the next 1/8 yard.

Fabric direction can change your choice

The smaller number is a useful comparison, not an instruction to rotate every fabric. Check a directional print before changing panel orientation. Decide where you want the lengthwise grain to run, particularly for a quilt that will hang on a wall.

This estimate assumes full-width rectangular panels. It does not optimize a pieced backing from scraps, match a print repeat, or add shrinkage. If any of those matter, keep the calculator's panel plan as a starting point and allow additional fabric separately.

Before you buy the backing

Write down the panel direction as well as the yardage. That small note makes it easier to remember why you chose one layout when you return to the project.

• Confirm the backing margin and seam allowance with your pattern or quilter.
• Measure the prepared fabric's usable width.
• Check print direction, grain, and any extra fabric needed for matching.
• Ask the shop to round upward further if it sells fabric in larger increments.