Use the seam allowance your pattern was drafted with, which is printed on the instruction sheet or the pattern pieces: 5/8 in (1.5 cm) on Simplicity, McCall's, Butterick and Vogue patterns and many others, 1 cm (3/8 in) or 1.5 cm on most indie and European patterns, and 1 cm or 6 mm (1/4 in) on knits. If you are drafting your own or the pattern has no allowance at all, 1.5 cm on woven fabric and 1 cm on knits is a safe default, narrowed to 1 cm or 6 mm on tight curves such as necklines, and widened to 2–2.5 cm (3/4–1 in) on seams you might want to let out.
What is a seam allowance and why does it matter?
The seam allowance is the strip of fabric between the stitching line and the cut edge. It exists so that the seam can be stitched without the edge fraying away, finished, pressed and, if necessary, let out. It is not decorative, and it is not optional: a pattern drafted with a 1.5 cm allowance that you sew at 1 cm comes out 1 cm bigger at every seam, which on a bodice with four vertical seams adds 4 cm (1 5/8 in) to the bust.
That is why the question matters more than it looks. The right allowance is whatever makes your stitching land on the line the pattern maker intended.
Which seam allowance do sewing patterns use?
| Pattern source | Usual allowance | Included in the pieces? |
|---|---|---|
| Big 4 (Simplicity, McCall's, Butterick, Vogue) | 5/8 in (1.5 cm) | Yes |
| Most North American indie patterns | 5/8 in or 1/2 in (1.5 or 1.2 cm) | Yes |
| Most European and Australian indie patterns | 1 cm or 1.5 cm | Yes, usually |
| Burda magazine patterns | None | No: you add it when tracing |
| Japanese pattern books | None | No: you add it when tracing |
| Knit and activewear patterns | 1 cm or 6 mm (1/4 in) | Yes |
| Quilting | 1/4 in (6 mm) | Yes |
The important column is the last one. A pattern with no allowance included must have it added before you cut, and a pattern that includes it must not have more added. The indie vs Big 4 vs AI patterns post goes into how the conventions differ, and the how to read a sewing pattern guide shows where the allowance is stated.
Which seam allowance for which seam?
Even within one garment, not every seam wants the same allowance. If you draft or adjust your own patterns, these are the conventional widths:
| Seam | Allowance | Why |
|---|---|---|
| Side seams, shoulder seams, center back | 1.5 cm (5/8 in) | Room to finish and to let out |
| Seams you may want to adjust at a fitting | 2–2.5 cm (3/4–1 in) | A toile or a first version of a fitted garment |
| Necklines, armholes, curved facings | 1 cm (3/8 in) or 6 mm (1/4 in) | Narrow allowances turn a curve cleanly with less clipping |
| Collars, cuffs, enclosed edges | 6 mm–1 cm (1/4–3/8 in) | Less bulk inside the enclosed seam |
| Knit seams sewn on a serger | 6 mm–1 cm (1/4–3/8 in) | The serger blade trims to its own width |
| Zip seams | 1.5–2 cm (5/8–3/4 in) | A wider allowance supports the zip tape |
| French seams | 1.5 cm (5/8 in) sewn in two passes | First pass 6 mm, trim, second pass 1 cm |
| Hems | 1–4 cm (3/8–1 5/8 in) | Narrow on curves, deep on straight hems |
How do you check which allowance your pattern uses?
Look at the instruction sheet first: most patterns state it in the first paragraph. On the pattern pieces, a line inside the cutting line marked 1.5 or 5/8 is the stitching line, and the distance between the two is the allowance. If the piece shows only one line and nothing is stated anywhere, assume it is the cutting line and that no allowance is included; this is the convention of Burda magazines and most Japanese books.
Two signs that an allowance is included: notches printed as triangles on the outer edge, and corners that look oddly pointed where two cutting lines meet. A sign that it is not: pieces that fit together exactly, edge to edge, with corners that look like the finished garment.
How do you sew an exact seam allowance?
Knowing the number is half the job; the machine has to hit it. The throat plate of most machines is engraved with lines at 1 cm, 1.5 cm and 2 cm, or 3/8, 5/8 and 3/4 in, measured from the needle in its center position. Keep the raw edge on the line, not the presser foot, since feet vary in width.
If your plate is unmarked or the line is hard to see, stick a strip of masking tape or a stack of sticky notes on the bed at the right distance, measured with a ruler from the needle. A magnetic seam guide does the same, and a quarter-inch foot sews a 6 mm allowance by keeping the edge against its flange. Measure your first seam with a ruler: 1.5 cm from the stitching to the edge, all along. The seam allowance converter turns inches to millimeters and back when a pattern and your machine use different units.
Can you change the seam allowance of a pattern?
Yes, and it is worth doing when you want to sew a Big 4 pattern with a serger, or add allowance to a Burda tracing. Draw a line parallel to the edge at the new distance with a clear ruler, or with a double tracing wheel set to the width. On curves, make a series of short marks with the ruler held at right angles to the edge, then connect them.
Two cautions. Change the allowance on every piece of the seam, not one side only. And keep the notches: move them outward or inward by the same amount, otherwise they no longer meet.
Narrowing a 1.5 cm allowance to 1 cm at a neckline before you sew saves trimming afterward, and it is the usual reason people adjust. Widening to 2.5 cm at the side seams of a fitted dress gives you a margin when you make a toile and want to let it out.
How much seam allowance on knits?
Knits do not fray, so the allowance only needs to hold the seam and to feed through the machine. Most knit patterns use 1 cm (3/8 in), and some use 6 mm (1/4 in), because a serger cuts to its own width anyway and a narrow allowance stretches with the seam better than a wide one pressed open. On a regular machine, sew a knit seam at 1 cm with a narrow zigzag or a stretch stitch, then trim to 6 mm if the fabric is thick. The sewing knit fabrics guide covers the stitches.
Sewly draws its patterns with 1.5 cm on woven fabric and 1 cm on knits by default and lets you change both figures before printing, so the allowance printed on the pieces matches what you plan to sew.
Common mistakes with seam allowances
- Adding allowance to a pattern that already includes it. The garment grows a size.
- Sewing along the edge of the presser foot without measuring what that distance is. On many machines it is 7 mm, not 1 cm.
- Using the same width on every seam of a self-drafted pattern. Necklines and armholes with 1.5 cm allowances need much more clipping to turn.
- Not trimming and grading enclosed seams. A 1.5 cm allowance inside a collar leaves a ridge unless each layer is trimmed to a different width.
When should you use a wider or narrower allowance?
Go wider, 2–2.5 cm, on the side seams and center back of anything fitted that you are sewing for the first time, and on fabrics that fray heavily, such as loosely woven linen and tweed. Go narrower, 6 mm to 1 cm, on knits, on tight curves, on sheer fabrics where a wide allowance shows through, and inside enclosed seams. A French seam on a sheer blouse is a special case: it needs 1.5 cm in total, used up over two passes. For everything else, the pattern's own figure is the right one, and the only job is to sew it accurately.