THE NUMBERS, EXPLAINED
How to use this calculator.
Compares vertical and horizontal straight-strip backing layouts and chooses the one requiring less fabric length; ties choose vertical. A join consumes twice the seam allowance. Trim excess width; no pattern matching or shrinkage allowance.
Start with your inputs
Enter quilt width (cm), quilt length (cm), overhang on every edge (cm), usable fabric width (cm), seam allowance on each joined edge (cm). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Quilt width (cm) | 150 |
| Quilt length (cm) | 200 |
| Overhang on every edge (cm) | 10 |
| Usable fabric width (cm) | 110 |
| Seam allowance on each joined edge (cm) | 1 |
A worked example
With the example inputs above, backing width (cm) is 170. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Backing width (cm) | 170 |
| Backing length (cm) | 220 |
| Fabric strips | 2 |
| Suggested strip direction | Vertical |
| Fabric length (m) | 4.4 |
| Straight joins | 1 |
How this is calculated
Backing width (cm)
Add overhang to both sides of the quilt width.
Backing length (cm)
Add overhang to both ends of the quilt length.
Fabric strips
For each orientation choose the smallest integer n with n × fabric width − 2 × seam allowance × (n−1) covering the required width.
Suggested strip direction
Choose the orientation with the smaller total strip length. Ties use vertical strips.
Fabric length (m)
Multiply chosen strip count by backing length along that orientation; divide by 100.
Straight joins
Strip count minus one.
Symbols and units
- a
- Quilt width (cm)
- b
- Quilt length (cm)
- c
- Overhang on every edge (cm)
- d
- Usable fabric width (cm)
- e
- Seam allowance on each joined edge (cm)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Compares vertical and horizontal straight-strip backing layouts and chooses the one requiring less fabric length; ties choose vertical. A join consumes twice the seam allowance. Trim excess width; no pattern matching or shrinkage allowance.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Backing width (cm); Backing length (cm); Fabric strips; Suggested strip direction; Fabric length (m); Straight joins. The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Compares vertical and horizontal straight-strip backing layouts and chooses the one requiring less fabric length; ties choose vertical. A join consumes twice the seam allowance. Trim excess width; no pattern matching or shrinkage allowance.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
OpenStax: geometric formulas and arithmetic ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
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