Quiltback / Calculation guide

Calculation guide

Quiltback estimates straight-strip backing from bolt fabric. It keeps target size, selvedge deductions, joining losses, end reserves and supplier rounding as separate steps.

The model: cut equal-length strips across the full usable fabric width, join their long edges, then trim the joined panel to the target backing dimensions. This is a geometric fabric estimate, not a quilting-method recommendation.

1. Turn the quilt top into a backing target

Enter the quilt top’s width and length. The extra backing input applies independently to all four edges. You choose the amount needed for your actual process; the tool does not prescribe a universal margin.

Target width = quilt width + 2 × extra at each edge
Target length = quilt length + 2 × extra at each edge

A 150 × 200 cm quilt top with 10 cm at every edge needs a 170 × 220 cm target backing panel.

2. Find the usable bolt width

Enter the full fabric width and the amount removed from each selvedge. The usable width is the full width less both deductions.

Usable strip width U = bolt width − 2 × selvedge deduction

A 110 cm bolt with 1 cm removed from each side leaves 108 cm of usable width. Use actual fabric measurements when precision matters; nominal bolt width and real usable width may differ.

3. Account for the joining seams

Joining two strips uses a seam allowance from each strip. One join therefore removes twice the entered allowance from the combined flat widths. For n strips and per-edge allowance s:

Joined span = n × U − 2 × s × (n − 1)
For more than one strip: n = round up [(required span − 2s) ÷ (U − 2s)]

If one usable strip is already wide enough, no joining loss is deducted. The tool requires U to be greater than 2s so that another joined strip adds positive coverage. Estimates are limited to 100 strips per seam direction.

4. Compare vertical and horizontal seams

Vertical seams run parallel to the quilt’s length. Strips cover the target width, and each strip’s finished length covers the target length. Horizontal seams run parallel to the quilt’s width; the roles reverse.

The comparison mode chooses the smaller rounded purchase length among available options. If these tie, it chooses the smaller raw length, then fewer seams, with vertical seams as the final tie-breaker. You can also require one direction. Fabric direction, motifs, grain and quilting requirements can rule out an arithmetic option.

5. Add end reserves, then round purchasing

Squaring reserve is added at each end of each strip. It does not change the desired finished backing size.

Cut length per strip = required along-seam dimension + 2 × end reserve
Raw bolt length = strip count × cut length
Purchase length = round up (raw length ÷ selling increment) × selling increment

Worked example in centimetres

Use the 170 × 220 cm target above, usable strip width 108 cm, seam allowance 1.2 cm on each joined edge and 0.5 cm reserve at each cut end. Two vertically joined strips cover 2 × 108 − 2 × 1.2 = 213.6 cm, enough for the 170 cm target width.

Each strip is 220 + 2 × 0.5 = 221 cm long. Two strips need 442 cm or 4.42 m of raw bolt length. If the supplier sells in 0.25 m increments, round up to 4.5 m. This adds 8 cm from purchasing increments.

The horizontal option needs three strips to cover 220 cm across. Each is 170 + 1 = 171 cm long, so raw length is 5.13 m, rounded to 5.25 m at the same selling increment. Comparison mode chooses the vertical option.

Worked example in inches and yards

A 60 × 80 in top with 4 in at every edge needs 68 × 88 in of backing. A 42 in bolt with 0.5 in removed at each selvedge leaves 41 in. With 0.5 in seams, two strips cover 81 in across. At 0.5 in end reserves, each vertical strip is 89 in long, so raw length is 178 in or 4.944… yd. Buying in 0.25 yd increments gives 5 yd.

What is outside this model?

The calculator does not plan diagonal piecing, added feature strips, pattern or motif matching, directional print layout, fabric shrinkage, offcut reuse or different fabric widths within one backing. It does not decide how much backing your chosen quilting method requires. Include any required preparation allowance deliberately in your own measurements.

Both seam directions assume equal-length full-width strips. The geometry may leave excess width after joining. The tool does not reclaim that offcut as another strip, and a smaller area total alone does not prove a usable cutting layout.

Units and saving

Changing centimetres to inches converts all project dimensions. Purchase metres or yards are a separate supplier choice. Changing that choice keeps the entered increment number, so 0.25 m becomes a 0.25 yd step. One yard is exactly 91.44 cm. Copy, download and print operate through your browser; they do not upload your inputs.