How to calculate quilt backing size

01 / FIND THE FINISHED SIZE

Start with the quilt top dimensions

Measure the actual quilt top after it has been pieced. If its edges vary, check the width and length in a few places and use the largest measurements so the backing can cover the whole top.

Then add the same overage to both sides of each dimension:

The factor of two matters because the extra is added on opposite sides. Adding four inches to each side means eight inches total across the width and eight across the length.

02 / WORK THROUGH AN EXAMPLE

A 60 × 72 inch quilt top

Suppose your quilt top is 60 inches wide by 72 inches long, and you plan for 4 inches of extra fabric on each side. The calculation is:

• Width: 60 + (2 × 4) = 68 inches
• Length: 72 + (2 × 4) = 80 inches

Your finished backing therefore needs to cover 68 × 80 inches before it is layered with batting and the quilt top. The extra gives your quilter room to hold the layers, quilt beyond the top edge, and trim the finished quilt. Ask your long-arm quilter or check your machine’s requirements before choosing an overage; the amount depends on how the quilt will be basted and quilted.

03 / KEEP TWO ALLOWANCES SEPARATE

Quilting overage is different from seam allowance

Overage is the extra backing beyond the quilt top’s edges. It stays outside the top while the layers are quilted and is usually trimmed after quilting. Choose it for your quilting setup, not from the width of a piecing seam.

Seam allowance is fabric inside a cut piece that is used to join it to another piece. It is consumed when you sew. A seam allowance does not add usable finished width to the backing, so a pieced back needs wider cut panels than the finished sections they create.

For example, a half-inch seam allowance on each of two joined edges uses one inch across that seam in total. The outside edges of this example do not need extra seam allowance unless you are joining them to something else.

04 / PIECE A SIMPLE BACK

Join two long strips to make 68 × 80 inches

To make the example backing from two vertical strips, cut each strip 34.5 inches wide by 80 inches long. Sew the 80-inch edges together with a half-inch seam allowance on each strip. The seam takes 0.5 inch from both pieces, leaving a finished width of 34.5 + 34.5 − 0.5 − 0.5 = 68 inches. The length stays 80 inches.

This is one straightforward arrangement, not the only way to piece the back. Check that your usable fabric is wide and long enough after removing selvages, and keep directional prints oriented the way you want them.

05 / USE SIZE EXAMPLES CAREFULLY

Common starting points in the planner

The planner includes these quilt-top examples. The backing column shows the result when you add 4 inches on every side. These are presets for getting started, not a universal size standard; patterns, desired drape, quilting methods, and personal measurements vary.

06 / CHECK BEFORE YOU CUT

Measure usable fabric and review the plan

Measure each rectangular piece after excluding selvages and any damaged area. Record width and length in consistent directions. If a print can be turned without affecting its design, you may allow a piece to rotate; directional prints often need to stay upright.

The planner makes a suggested rectangular layout from the pieces you enter. It can use at most one cut panel from each stash piece, may continue a row or column with another piece, and marks uncovered areas as additional fabric. It does not reuse offcuts, and its suggested layout is not a guarantee of the least fabric or fewest seams.

Before cutting, compare the target with your quilter’s backing requirements, confirm print direction, and check the cut list and assembly notes. If you want to explore your own dimensions and fabric pieces, open the quilt backing planner.