The seam allowance is the space between the seamline and the edge of the pattern or the seamline and the cut fabric edge. After the seams are stitched, the seam allowance is hidden unless the garment has exposed seams, which will then show on the correct side of the garment – this is referred to as a deconstructed look. The seam allowance protects the stitches from fraying. The seam allowance allows the garment to be fitted; the seam allowance can be stitched wider if the garment is too big, or let out if too tight. Seam allowance can be added in inches (imperial measurements) or in centimeters (metric measurements).
If you have your own design business, you’ll need to set a standard seam allowance for everyone to use. When seam allowances keep changing, it confuses the machinists. Whatever seam allowance you decide to use, for your company or in school, the important thing is to keep it consistent.
Adding Seam Allowance to the Pattern
When seams are enclosed or shaped, as for a curved neckline or collar seam, allow ¼-inch seam allowances. It is far easier to stitch narrower seam allowances around curved seams. If a ½ -inch seam allowance were to be used for an armhole or neck opening, it would need to be trimmed back to ¼-inch to reduce bulk, and this is a waste of time. The seam can still be carefully clipped, graded (depending on the thickness), and under-stitched before turning.
Add ½-inch for all seams: side, shoulder, armhole, waist, princess seams, yokes, and any other seams not mentioned. Add 3/4-inch seam allowance at center back and for any other seams where a zipper will be stitched. If the garment has a side seam zipper (a side seam normally has a 1/2-inch seam allowance), then make a step in the seam allowance to allow ¾- inch for the zipper to be stitched as well as the 1/2-inch side seam.
The fabric layout for a one-way fabric design illustrates how the step looks on the side seam. For fitting purposes, add wider seam allowances. Stretch knit garments only need 1/4-inch seam allowances, as knits do not fray. Most knit garments are stitched with a serger, and ¼-inch is the perfect width. For firm knits such as wool double knit, use 1/2-inch seam allowances for woven fabric widths.
The hem allowance is the width between the hemline and the hem edge. The hem allowance is folded back under the garment to the wrong side of the fabric; the clean finished edge is the finished hemline. There are times that the designer leaves a raw deconstructed hem edge as a design detail. When stitching hems, the fabric and garment silhouette determine the width of the hem allowance. Wide, bulky hems look thick and ugly and show a ridge from the correct side of the garment; this does not give a quality finish to the garment. In general, the wider and fuller the skirt, the narrower the hem width needs to be. This is how bulk is reduced. The following tips will help define the hem allowance used for different garment silhouettes.
Straight skirts made in a medium to heavy-weight fabric can have 1 1/2-inch to 2-inch hem allowances. A-line skirts have a wider silhouette, so reduce the hem allowance to 1 ½ inches to reduce bulk. A flared skirt is wider again, so reduce the hem allowance to 1 inch. A full-circle skirt is full and flouncy: reduce the hem width to 1/2 inch so bulk will not be a concern. In sheer fabrics a narrow hem will not shadow and will look inconspicuous from the correct side of the fabric. Hem allowances in knits are reduced to ½ to1 inch regardless of the style.
The amount of seam allowance added is important to achieving quality stitching; incorrect seam allowance will result in badly stitched seams.