How much seam allowance?
Seam allowance is the extra you add outside the finished (seam) line for sewing. The right amount varies by area. Here's a quick reference and tips for adding it cleanly.
1. What seam allowance is (two lines)
A pattern has two outlines: the seam line (the finished shape) and, outside it, the cutting line (where you cut). The gap between them is the seam allowance.
In Katagami you can set the seam allowance with a slider (0–3 cm), show or hide the seam line, and the pattern is output with the allowance included — no need to add it by hand.
2. Recommended amounts by area
| Area | Amount | Note |
|---|---|---|
| Straight seams (sides, shoulders) | 1.0–1.5 cm (3/8–5/8 in) | Standard; press open or to one side |
| Hems, cuffs | 2–4 cm (3/4–1 1/2 in) | Extra for folding under |
| Neckline / armhole curves | 0.7–1.0 cm (1/4–3/8 in) | Narrow is easier to finish |
| Zipper insertion | 1.0–1.5 cm (3/8–5/8 in) | Match the tape width |
| Bag / pouch openings | 1–3 cm (3/8–1 1/4 in) | Depends on the fold-over |
| Tight curves | 0.5–0.7 cm (1/4 in) | Clip to turn smoothly |
✏️ Beginners: using 1.0–1.5 cm everywhere is easy to remember and forgiving.
3. How to add seam allowance
- By hand: draw a line parallel to the seam line, the chosen distance outside it, on every edge. A grid ruler helps.
- Corners: extend the lines to their intersection, or fold the excess in after sewing.
- Automatically: in Katagami, just pick a value and an even allowance is added all around.
4. Clean curves and corners
- Outer curves (armholes): clip small notches into the allowance so it doesn't pull when turned.
- Inner curves: cut small V-notches out before turning to reduce bulk.
- Corners: trim the corner of the allowance diagonally before turning for a sharp point.
Frequently asked questions
- Should the seam allowance be the same everywhere?
- Use 1–1.5 cm on straight seams and more (2–4 cm) on hems for folding. If unsure, 1.5 cm everywhere works fine.
- Do I cut on the seam line or the cutting line?
- Cut the fabric on the outer cutting line; sew on the inner seam line. Katagami can show both.
- Same allowance for knits?
- Knits stretch, so 0.7–1.0 cm is easy to handle, especially if you finish with a serger.
Draft a pattern in your own size and print it at actual size on Letter or A4.
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