Definition
Seam allowance is the strip of fabric between the cutting line and the stitching line, usually 1.5 cm (5/8 in) on woven garment patterns.
The seam allowance is the margin outside the seam: you cut on the outer line, stitch on the inner one, and the strip between them ends up hidden inside the garment. It gives the seam something to hold onto, room to finish the raw edges and a little reserve for fitting.
Common widths:
| Width | Where you see it |
|---|---|
| 1.5 cm (5/8 in) | Most woven garment patterns |
| 1 cm (3/8 in) | Knits, many indie and European patterns, necklines |
| 6 mm (1/4 in) | Quilting, some knit patterns, tight curves |
| 2–2.5 cm (3/4–1 in) | Zip openings and side seams you may want to let out |
Always check the pattern before cutting. Some patterns, especially magazine patterns, are printed without allowances, and you add them yourself around every stitching line. On patterns that include them, guide the cut edge along the matching line engraved on your machine's needle plate and the seam lands where it was drafted.
Sewly patterns include the allowance: 1.5 cm (5/8 in) on woven fabric and 1 cm (3/8 in) on knits by default, and you can change it. To switch between metric and imperial widths, use the seam allowance converter; for how allowances appear among other pattern lines, see how to read a sewing pattern.