Enter the finished size of the piece, the seam allowance and the number of pieces. The cut size is the finished size plus two seam allowances, one on each side. Quilters typically use a quarter inch, garment patterns commonly use five eighths of an inch and some use a half inch.
Use the allowance your pattern states, and cut and sew with the same allowance for every seam.
Seam allowances vary by pattern and technique. Pressed seams and thick fabrics take up some width. Curved seams and hems have different allowances.
The area figure ignores waste and cutting layout, so add extra fabric.
Cut size per piece (inches) from finished piece size
| Finished piece size | 4 |
|---|---|
| Seam allowance | 0.25 |
| Number of pieces | 20 |
| Cut size per piece (inches) | 4.5 |
| Total fabric area needed (sq ft) | 2.8 |
Common error: Adding the allowance on one side only. It applies to every edge that is stitched, so the cut size grows by twice the allowance.
Seam Allowance: formula
Seam Allowance: changing Finished piece size
| Finished piece size | Cut size per piece (inches) | Total fabric area needed (sq ft) |
|---|---|---|
| 3.2 | 3.7 | 1.9 |
| 3.6 | 4.1 | 2.3 |
| 4 (example) | 4.5 | 2.8 |
| 4.4 | 4.9 | 3.3 |
| 4.8 | 5.3 | 3.9 |
Seam Allowance: compare input ranges
| Input | Tested from | To | Cut size per piece (inches): low input | Cut size per piece (inches): high input | Swing |
|---|---|---|---|---|---|
| Finished piece size | 3.6 | 4.4 | 4.1 | 4.9 | 0.8 (18%) |
| Seam allowance | 0.225 | 0.275 | 4.5 | 4.6 | 0.1 (2.2%) |
| Number of pieces | 18 | 22 | 4.5 | 4.5 | 0 |
Seam Allowance: individual input comparisons
| Input | Value | Cut size per piece (inches) | Total fabric area needed (sq ft) |
|---|---|---|---|
| Finished piece size | 3.2 | 3.7 | 1.9 |
| 3.6 | 4.1 | 2.3 | |
| 4 | 4.5 | 2.8 | |
| 4.4 | 4.9 | 3.3 | |
| 4.8 | 5.3 | 3.9 | |
| Seam allowance | 0.2 | 4.4 | 2.7 |
| 0.225 | 4.5 | 2.8 | |
| 0.25 | 4.5 | 2.8 | |
| 0.275 | 4.6 | 2.9 | |
| 0.3 | 4.6 | 2.9 | |
| Number of pieces | 16 | 4.5 | 2.3 |
| 18 | 4.5 | 2.5 | |
| 20 | 4.5 | 2.8 | |
| 22 | 4.5 | 3.1 | |
| 24 | 4.5 | 3.4 |
Cut size per piece (inches): Finished piece size against Seam allowance
| Finished piece size / Seam allowance | 0.2 | 0.25 | 0.3 |
|---|---|---|---|
| 3.2 | 3.6 | 3.7 | 3.8 |
| 3.6 | 4 | 4.1 | 4.2 |
| 4 | 4.4 | 4.5 | 4.6 |
| 4.4 | 4.8 | 4.9 | 5 |
| 4.8 | 5.2 | 5.3 | 5.4 |
Seam Allowance: values and definitions
| Quantity and definition | Value used |
|---|---|
| Finished piece size — Finished (sewn) size of one square patch, in inches. | 4 |
| Seam allowance — Seam allowance added to each side, in inches. | 0.25 |
| Number of pieces — Total number of patches needed. | 20 |
| Cut size per piece (inches) | 4.5 |
| Total fabric area needed (sq ft) | 2.8 |
Finished piece size: half and double
| Finished piece size | Cut size per piece (inches) |
|---|---|
| 2 | 2.5 |
| 4 | 4.5 |
| 8 | 8.5 |
A seam allowance is the fabric between the stitching line and the raw edge, and it is lost inside the seam. A patch that finishes at 4 inches with a quarter inch seam on each side must be cut at 4.5 inches. Twenty of them need about 2.8 square feet of fabric.
Cutting to the finished size makes every patch too small, and the whole quilt or garment comes out smaller than planned.
Use the seam allowance the pattern names and stitch accurately to it. A scant seam and a full one differ enough to change the fit across a whole quilt top.
Estimate fabric on the fabric yardage calculator, quilt backing on the quilt backing and binding page and resizing on the pattern scaling calculator. The yarn and knitting pages cover other crafts, and the square footage and rectangle area calculators help with areas.
What is a seam allowance?
The extra fabric between the cut edge and the stitching line, which lets the seam hold and not fray.