Seam allowance is the area between the intended stitching line and the raw cut edge. It provides the material a sewing machine needs beside the finished shape to join two pieces securely. The seamline determines the finished dimensions; the allowance normally remains inside the item.
Is seam allowance included?
Pattern companies follow different conventions. On some sewing patterns, the outer line is the cutting line and seam allowance is already included. Other patterns show the seamline, requiring an allowance to be added before cutting.
Always check the instructions and pattern legend. Adding allowance twice makes the project larger than drafted. Forgetting allowance when it is not included removes necessary fabric and makes the project smaller.
Common seam allowance widths
There is no universal width. Common examples include:
- 5/8 inch on many commercial garment patterns in the United States
- 1 or 1.5 cm on many metric patterns
- narrower allowances for selected fine edges or industrial and serger methods
- wider allowances at hems or fitting seams
- 1/4 inch for many patchwork projects
The correct amount depends on the pattern, fabric, and construction method. A French seam needs enough total allowance for both passes, while curved areas are often trimmed narrower after stitching.
Adding and sewing an even allowance
If allowance is not included on a pattern piece, add it with a seam gauge, ruler, or parallel marking tool. Around curves and corners, the new cutting line has to remain a true offset from the seamline; simply extending straight edges can distort the shape.
Needle-plate guides or an edge guide help keep the raw edge at a consistent distance while sewing. The goal is to stitch on the intended seamline, not merely to leave an even-looking strip afterward.
Finishing after stitching
Depending on the fabric and method, allowances may be pressed open, pressed together to one side, graded to reduce bulk, or finished with a zigzag stitch or serger. Inner curves are clipped and outer curves notched so they can spread or compress when turned. These cuts must stop before the seamline.
Seam allowance is therefore part of the engineering of a seam, not just surplus fabric. Its width and finish affect strength, flexibility, bulk, and how smoothly the finished area lies.