Quilt Backing Calculator

Backing size, seams and fabric width

The finished backing target starts with the quilt top and adds the entered margin twice in each direction:

target width = quilt width + 2 × extra per side

target length = quilt length + 2 × extra per side

APQS commonly illustrates a backing 4 inches larger on every side, but requirements vary. Dense quilting, lofty batting and a particular longarm frame can need a different margin, so the calculator keeps it visible and editable.

For joined full-width panels, every seam consumes an allowance from both raw edges. If n panels use fabric width F and seam allowance S, their joined span is:

joined span = n × F − 2 × S × (n − 1)

When shrinkage is entered, F is the estimated usable width after prewashing; the seam allowance is then applied unchanged when the prepared panels are joined.

With vertical seams, panels run along the quilt length and must cover the backing width. With horizontal seams, they run across the quilt width and must cover the backing length. The calculator finds the smallest whole panel count that covers each direction, then multiplies it by the panel cut length. It never rounds intermediate dimensions.

Worked example

A 60 × 80 inch quilt with 4 inches extra per side needs a 68 × 88 inch backing. Using 42-inch usable fabric and a ½-inch seam allowance, two vertical panels finish 83 inches wide. Each is cut 88 inches long, so the exact requirement is 176 inches, or 4.889 yards; rounded to the next ⅛ yard, buy 5 yards. A horizontal layout needs three 68-inch cuts, or 5.667 yards, rounded to 5¾ yards. Vertical uses less fabric in this example.

Optional shrinkage models fabric that is prewashed before the panels are joined. It reduces the estimated usable width after washing and increases the raw length bought before washing. This can change the panel count near a width boundary, not just add a little yardage. Real shrinkage can differ by fiber, finish, wash method and fabric direction; Moda lists tolerances for its fabrics rather than a universal rate. Test or measure the actual fabric when precision matters.

Practical limits

• Enter usable width after selvages, not the nominal width on the bolt.
• Remove selvages before joining. APQS recommends a ½-inch backing seam and pressing seams open, but follow your project or quilter’s instructions.
• Directional prints, large repeats, motif matching, defects and miscuts can require additional fabric. Shop-increment rounding alone is not a waste allowance.
• The lower-yardage plan is only a material comparison. Grain, hanging direction, seam placement and frame loading can justify the alternative.

See the APQS backing yardage chart, APQS guidance on piecing quilt backs and the Moda fabric care guide.