A seam allowance is the strip of fabric between the cut edge and the stitching line. It’s the part of every piece you never see on the finished project — and getting it wrong is the single most common reason blocks don’t line up and garments don’t fit.
Why different projects use different widths
There’s no universal seam allowance. Each width is a trade-off between precision, bulk, and room to adjust.
| Seam allowance | Where you’ll see it | Why |
|---|---|---|
| ¼” (6 mm) | Quilting, patchwork | Narrow seams keep bulk down where many seams meet, and quilt blocks don’t need fitting room |
| Scant ¼” | Precise piecing | A thread or two narrower than ¼”, to make up for the fabric lost in the fold when you press |
| ⅜” (1 cm) | Many modern and European patterns | A balance between bulk and room to finish edges |
| ½” (1.2 cm) | Bags, home decor, some indie patterns | Enough room for overlocking or zigzag finishing |
| ⅝” (1.5 cm) | Most commercial garment patterns (Big 4) | Room to let out side seams, finish raw edges, and adjust fit |
| 1” or more | Center backs, tailoring | Generous allowances you can let out later |
Always check the pattern: the seam allowance is printed on the instructions or the pattern pieces, and it’s usually the same for every seam unless stated otherwise.
Finished size vs. cut size
The finished size is what you see after sewing. The cut size is finished size plus the seam allowance on each side that gets sewn.
For a square with seams on all four sides:
- Cut size = finished size + 2 × seam allowance
- A 4” finished square with ¼” seams → cut 4½”
- A 4” finished square with ⅝” seams → cut 5¼”
Going the other way, subtract twice the allowance: a 6½” cut square with ¼” seams finishes at 6”.
The most common slip is adding the seam allowance once instead of twice. Every sewn edge needs its own allowance.
Why your ¼” seam might be lying
If your quilt blocks keep coming out small, your seam is probably a hair too wide. A seam just 1/32” too wide takes 1/32” from each piece, so every seam costs 1/16” — across a block with eight seams, that’s ½” gone. Two fixes:
- Use a ¼” foot with a guide, and test it: sew three 1½” strips together, press, and the middle strip should measure exactly 1”.
- Sew a scant ¼” — a thread’s width inside the line — to make up for the fold.
Convert without the mental math
The Seam Allowance Calculator converts finished to cut size (and back) for any allowance, including a scant quarter, in inches or centimetres. For quilt units like half-square triangles, which need extra on top of the seam allowance because of the diagonal seam, use the Half-Square Triangle Calculator.