Seam allowance converter: centimeters and inches
Type a seam allowance in centimeters or inches: you get it in the other unit, as the fractions on your sewing gauge, with the closest common allowance and how much they differ.
In inches: 0.625 or 5/8.
Conversion
Each seam joins two edges: 0.9 mm per edge changes the garment by 1.8 mm per seam.
Common seam allowances
| Metric | Imperial | Imperial in mm | Typical use |
|---|---|---|---|
| 6 mm | 1/4 in | 6.4 mm | Serged knit seams, quilting and patchwork |
| 1 cm | 3/8 in | 9.5 mm | Knits on a sewing machine, many European patterns |
| 1.5 cm | 5/8 in | 15.9 mm | Woven garments: the usual standard |
| 2 cm | 3/4 in | 19.1 mm | Seams you may want to let out, some zips |
| 2.5 cm | 1 in | 25.4 mm | Hems, fitting room on a toile |
The common seam allowances pair up like this: 6 mm and 1/4 in, 1 cm and 3/8 in, 1.5 cm and 5/8 in, 2 cm and 3/4 in, 2.5 cm and 1 in. They are close but not equal: 5/8 in is 15.9 mm, almost 1 mm more than 1.5 cm. To convert exactly, divide centimeters by 2.54 to get inches, or multiply inches by 2.54.
Seam allowance conversion chart
| Metric | Imperial | Imperial in mm | Difference | Typical use |
|---|---|---|---|---|
| 6 mm | 1/4 in | 6.35 mm | 0.35 mm | Serged knit seams, quilting and patchwork |
| 1 cm | 3/8 in | 9.5 mm | 0.5 mm | Knits on a sewing machine, many European patterns |
| 1.5 cm | 5/8 in | 15.9 mm | 0.9 mm | Woven garments: the usual standard |
| 2 cm | 3/4 in | 19.05 mm | 0.95 mm | Seams you may want to let out, some zips |
| 2.5 cm | 1 in | 25.4 mm | 0.4 mm | Hems, fitting room on a toile |
The seam allowance is the distance between the cutting line and the stitching line. A 6 mm (1/4 in) allowance is standard on a serger, which trims and finishes as it sews, and in quilting; 1.5 cm (5/8 in) is what most commercial patterns use for woven garments.
When does the difference matter?
Each seam joins two edges, so whatever you change per edge counts twice in every seam. If you sew 1.5 cm on a pattern drafted with 5/8 in, each seam lets out about 1.8 mm. Around a skirt with four seams (two side seams, center front and center back) the waist grows by about 7 mm (1/4 in): within the ease of most garments, though it adds up on a fitted bodice with many seams.
The difference matters more when the allowances themselves differ. Sewing 1 cm (3/8 in) on a pattern made for 1.5 cm (5/8 in) lets out 1 cm (3/8 in) per seam, so 4 cm (1 5/8 in) around that same skirt: enough to change the fit. Sew the allowance the pattern was drafted with, and keep the edge of the fabric on the right line of your machine's throat plate, or on a strip of tape stuck at the right distance.
Does my pattern include seam allowances?
Check before you cut. Most printed and PDF patterns include them and say so on the pattern pieces or in the instructions; some drafting books and magazine patterns do not, and you add them when cutting. How to read a sewing pattern shows where to look.
Sewly's patterns include them: 1.5 cm (5/8 in) on woven fabric and 1 cm (3/8 in) on knits by default, and you can change them before printing.
Frequently asked questions
Is 1.5 cm the same as 5/8 inch?
Almost: 5/8 in is 1.59 cm, 0.9 mm more. On one seam you will not see it; on a garment with many seams it changes the fit by a few millimeters, so sew the allowance the pattern was drafted with.
What is the standard seam allowance?
1.5 cm (5/8 in) for woven garments on most commercial patterns, 1 cm (3/8 in) or 6 mm (1/4 in) for knits, and 6 mm (1/4 in) in quilting. The allowance is printed on the pattern or in its instructions: always check.
How do I convert inches to centimeters?
Multiply by 2.54. For a fraction, divide first: 5/8 in = 0.625 in, and 0.625 × 2.54 = 1.59 cm. In the other direction, divide centimeters by 2.54.
Can I change the seam allowance of a pattern?
Yes, as long as you know the one it was drafted with. Add or trim the difference along the cutting line, then sew on the original stitching line so the finished size stays the same.