Pattern piece C
Seam allowance converter
List the edges of your pattern with the allowance they have and the one you want; see exactly how much to add or trim at each cutting line, in inches and millimetres.
How to change a pattern's seam allowances
A pattern's instruction sheet says which allowance its pieces include, and it is not always the same on every edge: necklines, hems and zip seams often differ from the main seams. Enter each kind of edge with the allowance it has now and the one you want. You can type fractions (5/8, 1 1/4), decimals, or a unit after the number, so “15 mm” works in the inch view.
Add means cut that much outside the printed cutting line; trim means cut inside it. Mark the new line with a ruler, or, for small changes, lay a seam gauge against the old line as you cut. Notches stay on the stitching line, so clip them a little shallower when you trim, and move any that would vanish.
For a pattern with no allowances, set “now” to 0 and choose the width you like to sew: the add column is the amount to draw around each piece.
Worked example: a pattern with 5/8 in seams that you want to sew at 3/8 in on an overlocker. Each side and shoulder edge loses 1/4 in (6.4 mm) at the cutting line. If you skip the trimming and simply sew at 3/8 in, every seam edge gives back 1/4 in, so a skirt with four seams round the body grows 2 in all the way round: the “different allowance” box shows that.
For metric work, 5/8 in is 15.9 mm and 1.5 cm is the nearest round figure; for most single seams the two are interchangeable. Small differences matter more on many-seamed pieces, such as six-gore skirts.
Common allowances in millimetres
One inch is exactly 25.4 mm, so the fractions on a seam gauge rarely land on whole millimetres. The last column rounds to the nearest half centimetre, a handy step for metric pattern work.
| Inches | Exact | Nearest mm | Nearest ½ cm |
|---|---|---|---|
| 1/8 in | 3.18 mm | 3 mm | 0.5 cm |
| 1/4 in | 6.35 mm | 6 mm | 0.5 cm |
| 3/8 in | 9.52 mm | 10 mm | 1 cm |
| 1/2 in | 12.7 mm | 13 mm | 1.5 cm |
| 5/8 in | 15.88 mm | 16 mm | 1.5 cm |
| 3/4 in | 19.05 mm | 19 mm | 2 cm |
| 1 in | 25.4 mm | 25 mm | 2.5 cm |
| 1 1/4 in | 31.75 mm | 32 mm | 3 cm |
| 1 1/2 in | 38.1 mm | 38 mm | 4 cm |
| 2 in | 50.8 mm | 51 mm | 5 cm |
Sources
- NIST, SI units – length: the inch is exactly 25.4 mm.
- New Mexico State University, T is for T-Shirts (Guide C-210): seam allowances vary from 1/8 to 5/8 inch depending on the pattern, and 5/8 in seams on knits may be trimmed to 1/4 or 3/8 in.
Questions
Does changing the seam allowance change the size?
Not if you move the cutting line and keep sewing on the pattern's stitching line. It does if you leave the cutting line alone and sew at a different allowance: every seam then moves, which is what the middle calculator measures.
Is 5/8 inch the same as 1.5 cm?
Nearly: 5/8 in is 15.875 mm, so 1.5 cm is about 0.9 mm smaller. On one seam that is lost in the stitching; across a skirt with eight seam edges it adds up to about 7 mm round the waist.
Why make the allowance narrower on necklines and armholes?
Curves are easier to sew accurately, and turn more smoothly, with less fabric beyond the stitching line. Many sewists cut curved edges at 1/4 or 3/8 in so there is little to clip or grade afterwards.
Can I trim allowances after sewing instead?
Yes, if you sew on the pattern's stitching line first. Changing the allowance before cutting matters most when you want to sew with an overlocker, which cuts and sews at its own width, or when the allowance has to fit a guide on your machine.