A Guide to Seam Allowances

Learning about seam allowance is an important step in sewing. Seam allowance differs from one project to another. It depends on certain factors, but it’s a fundamental part of any sewing venture and keeps your seams strong and dependable. Accurate seam allowances create proper pattern piece alignment and avoid improper fabric cuts.

1. What is Seam Allowance?

Seam allowance is the area between the cutting edge and the seam line that joins 2 (or more) pieces of fabric together. It is usually hidden from the outside and visible from the inside of the garment. Though some seams have exposed seam allowances, such as flat-felled seams for decorative purposes. Seam allowance is dependent on the fabric, pattern, or design.

2. Industry and Commercial Pattern Standard Measurements

Commercial patterns commonly specify seam allowance, but seam allowance can range from ¼” to ⅝”. The variation in seam allowance is often due to the weight of the fabric. Typically a larger seam allowance is used for heavier fabrics, while a smaller seam allowance is used for lightweight fabrics. It is the measured difference in width from the raw edge to the seam that will be stitched.

The industry standard for SA is different due to the machinery used and doesn’t follow the same guidelines as home sewing methods. 

A ⅝” seam allowance is often considered the commercial pattern standard. It offers room from the edge to the stitching line to ensure each layer is joined together. For fabric that unravels easily, this seam allowance amount keeps stitches from fraying off the edge.  

Areas with curves, outside edges, welt pockets, faced areas, and sleeveless armholes often have a ¼” seam allowance. It’s easiest to use this measurement when sewing around small, curved spaces.

General seams, which make up most of the seams that haven’t been listed above fall into SA measurements of ⅜”, ½”. And ⅝”. When drafting patterns, SA is often left to the designer’s discretion, so choose the one that’s most practical for your garment. 

Bound shirt sleeves and plackets use a ⅛” SA because these areas are very narrow, and a SA allowance larger than this would create bulk in the area and would require trimming. 

SA for hems also varies based on the garment and technique used and typically ranges from ¼” -3″. Hems typically have at least two folds, and each fold will vary in range. Allow 1″ for the hem allowance plus standard seam allowance of ⅝”. 

3. Adding Seam Allowance/Tools

If you’re drafting a pattern, a clear ruler is an excellent tool for drawing and measuring SA. To transfer SA to fabric, use a tracing wheel and tracing paper or use tailor’s chalk. You can also use a fabric pen to mark and measure the SA lines on the fabric.

4. Throat plate/needle plate guide

Use the throat plate/needle plate as a guide for fabric. It has grooves and measurements to help you line up your fabric properly as you sew.

• Set the needle in the center, then use the plate markers to get an accurate seam allowance.
• For a ¼” presser foot, line up fabric to the right of the presser foot for accurate seam allowances.
•  No seam allowance guide? No problem. Create your guide by using masking tape or painter’s tape as a reference guide, then line up fabric as directed above.

5. Tips

• Any major alterations to a pattern should be made before it is cut to avoid unintended changes to the overall finished product. 
• Seam allowance does allow room for minor alterations/adjustments.
• Press open seam allowance using pressing tools to fold the fabric back on itself exposing the sewn seam. 
• Read through patterns to determine seam allowance before buying fabric.
• Commercial patterns with multiple sizes come with lines to cut that already include seam allowances.
• Check the throat plate for accuracy by using a sewing gauge to check the correct line.
• Pin into SA to avoid holes in your fabric.
• Clip into SA using fabric sheers around curved edges to smooth and reduce bulk.
• Add SA around the entire pattern except for any cut on fold sections.

Seam allowance creates a better-finished product, but it isn’t always cut and dry, so always pay attention to SA directions and do your research if SA isn’t specified or you are drafting your patterns. Accurate seam allowances cut down on added stress and make an overall better garment.