Seam allowances can be easy to overlook since they are hidden and outside the silhouette of the finished project, but seam allowances dictate whether your pattern pieces can be sewn into the final shape. If a pattern instructs you to stitch x-in. from the raw edge, that amount is not an extra buffer to make up along the way; that is your exact stitching distance. That is how you connect pieces; that is how you maintain consistent seam widths. Your seam allowance ensures your hems, facings, waistbands, side seams, and more won’t end up smaller than the pattern intended.
A garment may still get wonky even when a beginner sews a perfect line down the center. The issue could be a shoulder that is sewn wider than the pattern instructs, thus decreasing the neckline size, or a side seam that is sewn too narrow, thus leaving the garment larger in certain areas. A hem may get uneven folds that look strange. These mistakes can be very subtle in the beginning, but you may not notice the cumulative impact across several seams until the garment is too late, especially when a mistake occurs on both sides of a single piece.
Before sewing anything, grab your pattern or make a quick sample of your project’s fabric and sketch the pattern’s seam allowances on one edge with tailor’s chalk or a fabric pen. Once you have set that seam allowance and lined up your fabric’s raw edge under your machine’s presser foot against the right seam guide, take it from there. Sew while focusing on the raw edge and your seam guide and not solely the needle. Afterward, measure the space between the raw edge and your stitching to gauge consistency in different places and whether you need to make any adjustments. You will begin to notice whether your hands are moving steadily or unintentionally wandering off.
Remember that you do not need to pull the fabric, as this can cause distortion on certain areas such as curved sections. Do not push either, as this creates unevenness and ripples that could be mistaken for gathering. Push your fabric forward in small increments, and use your fingers as much as possible to control your sewing path. Your sewing machine is a tool to move your fabric forward, and your fingers are there for support. If you catch that the seam allowance shrinks, stop and reset with your needle down before continuing to sew. It is better to stop and reset with your needle down than to push through a long seam and try to control the fabric.
Another reason to care about your seam allowances is that seam allowances can teach you how to read pattern marks. Darts, notches, and grainlines are placed to guide you toward where to sew each connection before your stitches are in place, and a fold line is where your fabric should sit. There may be one piece that looks to be one way or another on either end, but you can see which side joins first when the seam allowance is included, not when notches and grainlines aren’t marked. This is an easy way for garment makers to make a mistake, since pieces may look out of place even if cut in proper measurements. Mark them properly to ensure your pieces are sewn correctly.
After stitching, pressing makes it easier to assess how your seams look. Sewing a seam open or to the side can alter the look of a project; grading or reducing bulky pieces can help you move past this problem; finishing raw edges with a zigzag or other treatment can help stop fraying. These are small things that are only visible or possible if your seam allowance remains even and not accidentally too wide due to an uneven stitch.
The last thing to check is to review your stitch lines before checking your project for fit. Check to see that both sides are lined up on the seam edges. Are your stitches the proper distance from your raw edge? Is the thread tension appropriate? Did the fabric pucker in a particular place? If you take the time to measure your seam allowances before your needles ever touch your fabric, you will begin to view garment construction as a logical and consistent process, step by step.