Seam Allowance Calculator
Enter a finished measurement and select seam allowance type to get the cut size.
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Request a ToolHow to Use the Seam Allowance Calculator
This calculator takes a finished pattern measurement and adds the seam allowance needed to get the cut size. Use it whenever a pattern gives you finished dimensions and you need to know how big to cut the fabric pieces.
- Enter the finished measurement. This is the dimension the sewn piece should be after sewing the seam. For example, if a pocket should be 5" wide when finished, enter 5.
- Choose the allowance type. Standard 5/8" is used in most commercial sewing patterns. Serged 3/8" is used for overlocked seams. French seam 1/2" is used for the first seam of a French seam construction. Narrow 1/4" is common for quilting. Use Custom for any other size.
- Select sides. Most seams have two sides (both pieces being joined). Select 1 side if you are only adding allowance to one edge (like a single hemline or facing attachment).
- Read the cut size. This is what to mark and cut on your fabric.
About the Seam Allowance Calculator
Seam allowance is the fabric between the stitching line and the cut edge. Most commercial patterns in the US include 5/8" seam allowance in the pattern pieces, meaning the pieces are already cut to the right size — you sew at the 5/8" mark. However, when drafting your own patterns or adjusting existing ones, you need to add allowance to your finished measurements. The standard for apparel is 5/8", for quilting is 1/4", and for serged ready-to-wear construction is 3/8".
Frequently Asked Questions
What is the standard seam allowance for sewing garments?
The standard seam allowance for garment sewing in the United States is 5/8 inch (about 16mm). This is the allowance built into most commercial patterns from brands like Simplicity, McCall's, Butterick, and Vogue. European patterns often use 1.5 cm (about 9/16") or 1 cm as their standard. Quilting patterns typically use 1/4" seam allowance, and overlocked seams are often sewn at 3/8".
Do commercial patterns already include seam allowance?
Yes, almost all commercial sewing patterns include seam allowance in the printed pattern pieces. The outer edge of the pattern piece is the cut line, and the inner line (usually dotted or dashed) is the sewing line. You do not need to add seam allowance to commercial patterns — just cut on the outer line and sew on the inner line. This calculator is most useful when drafting your own patterns from finished measurements.
What is a French seam and how much allowance does it need?
A French seam encloses the raw edges within the seam, creating a clean finish on both sides. It requires two seams: sew wrong sides together first with a 1/4" seam, trim to 1/8", press, then fold right sides together and sew a 3/8" seam to enclose the first. The total allowance consumed is 1/2". Use the French seam setting in this calculator when you need the cut size for a French seam construction.
How much seam allowance does quilting use?
Quilting almost universally uses a 1/4" seam allowance, which is the narrow setting in this calculator. This smaller allowance is used because quilt blocks have many seams, and the bulk adds up quickly. Most modern sewing machines have a 1/4" quilting foot available. Pressing seams open rather than to one side also reduces bulk. When calculating the cut size of quilt squares, add 1/2" total (1/4" per side, two sides) to the desired finished size.