Seam Allowance Converter
Convert seam allowances between inches and millimetres - 5/8", 1/2", 3/8", 1 cm, 1.5 cm and more - plus the seam allowance conventions used by different pattern companies, and how to add allowances to patterns that don't include them.
Published
Common seam allowances
| Imperial | Exact metric | Usual metric equivalent | Where you'll see it |
|---|---|---|---|
| 1/4" | 6.35 mm | 6 mm | Overlocked knit seams, quilting, necklines and armholes in some patterns |
| 3/8" | 9.5 mm | 1 cm | Many independent patterns, especially for knits |
| 1/2" | 12.7 mm | 1.3 cm (sometimes 1.2) | Many independent patterns |
| 5/8" | 15.9 mm | 1.5 cm (sometimes 1.6) | The standard on most US commercial patterns |
| 3/4" | 19.1 mm | 2 cm | Some side seams, to allow for fitting |
| 1" | 25.4 mm | 2.5 cm | Fitting allowances on muslins, centre back seams, some hems |
| 1 1/4" | 31.8 mm | 3 cm | Hems on skirts and trousers |
| 1 1/2" | 38.1 mm | 4 cm | Deep hems on straight skirts and coats |
| 2" | 50.8 mm | 5 cm | Very deep hems; children's clothes made to be let down |
Why the small difference matters
Metric equivalents are rounded, and the rounding is usually harmless. But it adds up. 5/8" is 1.59 cm, not 1.5 cm. If you sew a pattern drafted for 5/8" allowances at 1.5 cm, each seam takes up 0.9 mm less on each side, so the garment gains about 1.8 mm of circumference per seam. A bodice with four vertical seams around the body (two side seams and two princess or back seams) grows by roughly 7 mm, about 1/4". That's rarely a problem on a loose top, but it's noticeable on a fitted bodice or a waistband. Use the allowance the pattern was drafted with, and check which markings on your machine's needle plate are inches and which are millimetres.
Seam allowance conventions
- US commercial patterns (the large pattern companies): 5/8" (1.5 cm) on most seams, with smaller allowances where stated, such as 1/4" on some necklines and facings.
- Independent pattern companies: vary widely, commonly 3/8", 1/2" or 1 cm. Always check the instructions; it's usually stated on the first page.
- Pattern magazines and many European and Japanese pattern books: often include no seam allowance. You trace the pattern and add allowances yourself.
- Knit patterns designed for an overlocker: often 1/4" or 3/8", matching the width an overlocker trims to.
- Pattern blocks and slopers: normally drafted without seam allowances, so that changes can be made on the true seamlines. See Basic Bodice Block.
Adding seam allowances to a pattern
- Trace the pattern onto paper, marking the seamlines, notches and grain lines.
- Measure out from each seamline by the allowance you want and mark it at short intervals. A clear ruler with a grid, a seam gauge, or a double tracing wheel (two wheels set a fixed distance apart) makes this quick.
- Join the marks into the cutting line. On curves, mark more frequently.
- At corners, extend the seamlines and cutting lines so that, folded along the seamline, the allowance matches the angle of the seam it will be sewn to. Folding the paper along the seamline and cutting the allowance while folded is the easy way to get these "jog" shapes right.
- Transfer the notches out to the new cutting line.
- Write the seam allowance on every piece. You won't remember later.
Some techniques need a particular allowance. A French seam splits it into two passes (1/4" plus 3/8" for a 5/8" allowance). A flat-felled seam works best with at least 5/8". Choose the allowance with the seam finish in mind.