Why Seam Allowance Matters Before You Cut Costume Pieces

Seam allowance refers to the distance between the cut edge of a pattern and the stitching line. It is important to mark seam allowance before cutting out costume pieces, because fabric disappears into the seam when pieces are sewn together. If you forget to mark seam allowance before cutting, or if you make seam allowances inconsistent throughout the garment, you are going to have problems.

Some commercial sewing patterns come with seam allowance included, meaning the pattern pieces show the actual cut size of the garment. Other patterns only show the true stitching line, and you will need to add seam allowance when you transfer the pattern pieces to fabric. Regardless, it is a good idea to mark the seam allowance with tailor’s chalk or a removable fabric marker before cutting with fabric shears. Seam allowance is especially crucial to mark for the following reasons:

Sewing by the seam line helps you produce consistent results. If your seam allowance is inconsistent, it is going to be difficult to guide your sewing machine by the raw edge of the fabric. As a result, the seam allowance will likely wander or become twisted, leading to tightness in some areas, inconsistent seam width, and hem lengths that don’t match. If you sew by the correct seam line, it is not necessary to match the uneven cut edges of the fabric.

Curved seams, such as necklines and armholes, can be affected by even minor deviations in seam allowance. If you cut the pieces incorrectly, they will likely not lie flat or fit together well in the seams. If you need to trim or clip curved seam allowances, you have to do that after the seam has been sewn. It’s not a good idea to trim the seam allowance early because you’ll weaken the edge, and you will have less margin to make any necessary alterations for fit.

You need a larger seam allowance if the seam might need to be let out after the fitting is completed. This is not an option if the seam allowance is cut too close.

Always be sure to include a fitting margin when cutting costume pieces to leave room for adjustments. While this is most important with woven fabrics and closely fitted bodices and sleeves, it is a good idea to always include room for movement and ease between layers.

Before making the final cut, double-check seam allowance along with pattern pieces, grainlines, and matching points. If you are unfamiliar with how seam allowance affects a seam, sew some sample rectangles or curved pieces using cheap fabric. First make the seam allowances even, and then uneven, and see how this can affect your finished pieces when you try to sew by the raw edges. Try using a seam gauge to draw seam allowances on your practice pieces. Draw the lines parallel, using an even distance, down the sides of rectangles to simulate working on the sides of a skirt, a sleeve edge, or the side seams of a dress.