What seam allowance does my pattern use?

Find it stated in the instructions before you cut anything, because it varies between patterns and some patterns do not include it at all.

The common conventions. Commercial paper patterns from the large companies traditionally use 15 mm on garment seams. Many independent designers use 10 mm. Some drafting books and most Japanese pattern books give net lines with no allowance included, so you add your own. None of these is more correct than another; what matters is knowing which you have.

How to tell. The instruction sheet states it, usually on the first page. If it does not, look for a cutting line and a separate stitching line on the pattern piece: two lines means the allowance is included. A single line on a piece with no stated allowance almost always means you must add it.

Why it matters more than it sounds. Sewing a 15 mm pattern at 10 mm adds 20 mm to the circumference at every seam. Across four seams that is 80 mm of unwanted width, and the fit problems it causes look like sizing problems.

Where the allowance changes within one pattern. Hems are usually deeper than seams. Necklines, armholes and any tight curve are often narrower — 6 mm — because a wide allowance cannot lie flat on a curve. Zip openings sometimes get extra.

Mark the allowance you are using on each piece as you trace, and set a seam guide on the machine to match. Judging 15 mm against the needle plate markings by eye drifts over a long seam.