How to Handle and Finish Seam Allowances
A seam allowance is the strip of fabric between the stitching line and the raw edge, and how you treat it decides whether a project lies flat or bulks up. This guide covers pressing, trimming, and finishing so your seams behave inside a finished garment.
Materials
- Fabric
- All-purpose thread
- Water-soluble marking pen
Tools
- Sewing machine
- Iron and pressing board
- Seam gauge or small ruler
- Sharp fabric scissors
- Pinking shears (optional)
Steps
- 01
Measure consistently
Most commercial patterns use a 1.5 cm seam allowance, so mark that line on your throat plate with tape if it is not already etched. Measure from the needle to the guide and keep every seam the same width.
- 02
Press the seam first
Set the seam by pressing it flat while the fabric is still right sides together, then open it with your fingers and press it open or to one side. Use steam and lift the iron rather than dragging it so the fabric does not stretch.
- 03
Choose open or to one side
Press seams open on structured garments like jackets and on bulky fabrics where you want the thickness spread out. Press to one side on lightweight fabrics and on curves, usually toward the back or the darker piece.
- 04
Trim and grade bulky seams
On thick fabrics, trim each allowance to a different width, for example 6 mm and 10 mm, so the layers do not stack up. Trim both layers together only on very light fabric where bulk is not a problem.
- 05
Clip and notch curves
Clip small V-shaped notches into the allowance on outward curves and straight snips on inward curves so the fabric can spread or close smoothly. Cut close to the stitching line but never through it.
- 06
Finish the raw edges
Serge, zigzag, or turn and stitch the raw edge so it does not fray through repeated washing. Finish each allowance separately when pressing open, and together when pressing to one side.
Standard allowances and where they come from
Most commercial garment patterns use 1.5 cm, and many North American patterns use 5/8 inch, which is very nearly the same. Quilting and bag patterns usually assume 1/4 inch or 6 mm, and knowing which convention you are in prevents a garment that comes out several centimetres too small.
Check whether the allowance is already included on the pattern pieces before you cut. Some pattern lines are drawn at the finished seam line and expect you to add the allowance yourself.
Guides, marks and consistency
Mark the allowance on the throat plate with a strip of tape at the distance you actually sew, measured from the needle rather than from the edge of the plate. Consistency matters more than the exact number, because every seam in a project inherits the same error.
For curved seams, draw the line on the fabric in chalk or washable pen. Following a drawn line is more accurate than trying to hold a curved edge against a straight guide.
Pressing open or to one side
Press seams open on structured garments such as jackets and on bulky fabrics where you want the thickness spread across the seam. Press to one side on light fabrics and on curves, usually toward the back or the darker piece.
Always set the seam first, pressing it flat while the two layers are still together and warm from the needle. Skipping the set is the most common reason a pressed seam looks slightly puffed.
Trimming, grading and clipping
Grading means trimming the two layers of a bulky seam to different widths, such as 6 mm and 10 mm, so they do not stack into a ridge. Trim the layer that will sit outermost in the finished garment the shorter of the two.
Clip V-shaped notches into the allowance on outward curves and straight snips into inward curves, cutting close to the stitching but never through it. Under-stitching the facing afterwards keeps the seam rolling neatly to the inside.
Finishing raw edges: which method when
A serger or overlock is fast and neat on fabrics that fray badly. A zigzag or three-step zigzag works on most machines and is adequate for casual garments, while a turned-and-stitched edge looks cleanest on light fabric that will be seen.
French seams enclose the raw edges completely and suit sheer or unlined garments, at the cost of a slightly bulkier seam. Pinking shears slow fraying on firmly woven cotton, but they are not a real finish for fabrics that shed.
Frequently asked questions
- What is a standard seam allowance?
- 1.5 cm, or about 5/8 inch, is standard for most commercial garment patterns. Quilting and bag patterns usually use 1/4 inch or 6 mm. Always check the pattern instructions, because some designs expect you to add the allowance yourself.
- Can I press a seam open on any fabric?
- Open pressing suits structured garments and bulky fabrics. On very light or loosely woven fabric, and on curves, pressing to one side gives a stronger and flatter seam.
- Do I need to finish every seam allowance?
- If the garment will be washed, yes, otherwise the edges fray and the seam eventually fails. For fabrics that do not fray, such as felt or some stable knits, a finish is optional.
- How do I stop thick seams from showing through on the outside?
- Grade the two layers to different widths, trim the corners, and press thoroughly at every stage. A clapper or a wooden block pressed onto the seam while it cools helps it set flat and thin.