Seam Allowances: What They Are and How Much Each Seam Needs

What you take from this lesson: Know how much seam allowance each seam needs, and tell whether a pattern includes it before you cut.

What a seam allowance is

It is the fabric that sits on the inside of the seam, between the cut edge and the line where the stitching runs. Without it, the seam would run exactly along the edge and the garment would come apart at the first tug.

The line of the finished garment is one line. The edge you cut along is another, a few millimeters or centimeters farther out. The distance between the two is the seam allowance.

Why it is not the same across the whole garment

The temptation is to put one centimeter everywhere. It does not work, for two reasons. First: large allowances get in the way on tight curves, because the extra fabric bunches up and forms folds on the inside. Second: some seams will be pressed open or sewn twice and need more fabric.

Common values in production:

• Straight seams (sides, shoulders, inseam): 1 cm.
• Armhole and neckline: 0.5 to 0.7 cm. They are tight curves, and a large allowance bunches up.
• T-shirt or sweatshirt hem: 2 to 2.5 cm, because it folds over on itself.
• Pants hem: 3 to 4 cm, especially if you want to leave extra allowance for lengthening later.
• Elastic waistband: the width of the elastic plus 1 cm.
• Seams that will be pressed open: 1.5 cm.

How to tell whether a pattern includes the seam allowance

It is the question to ask before cutting, because getting it wrong costs the entire production run. A pattern with the allowance included is cut along the outer line. One without it has to be enlarged by hand before it goes on the fabric.

The signs that the allowance is included: the pattern says so explicitly, you see two parallel lines along the edges (the cutting line and the stitching line), or the notches are drawn on the outer edge.

Moldey sewing patterns always come with the seam allowances included: you cut along the outline, without adding anything.

Seam allowance and fit

A detail that surprises many people: the seam allowance does not change the fit of the garment, as long as you sew where you are supposed to. The garment measures what the finished-garment line measures, not the cutting line.

That is why what ruins the fit is not the allowance itself, but sewing with a different allowance than the one the pattern planned for. If the pattern has one centimeter and the workshop sews at a centimeter and a half, every seam takes an extra half centimeter. On a garment with four vertical seams, that is two centimeters less circumference.

Common mistakes

• Cutting along the inner line on a pattern that already included the allowance: the garment comes out small.
• Adding allowance to a pattern that already had it: the garment comes out big and the fit is lost.
• Sewing with a different allowance than the one planned and blaming the pattern for the fit.

In short

The seam allowance is the fabric between the cut edge and the stitching. It is not the same across the whole garment, and the first thing to confirm before cutting is whether the pattern includes it.