What Is Seam Allowance?
Seam allowance is the distance between a fabric’s cut edge and the stitching line. It gives you material beyond the finished seam so two pieces can be joined, pressed and finished without changing the intended finished dimensions.
The correct amount depends on the pattern, sewing method, fabric and project. There is no single seam allowance that is correct for every project, so always check the pattern instructions before cutting.
Seam Allowance Calculator
The calculator above works in both directions. Use Finished size → cut size when you know the dimensions you want after sewing and need to add allowance. Use Cut size → finished size when a piece already includes seam allowance and you want to find the dimensions inside the stitching lines.
For rectangular pieces, each selected left or right edge changes the width by one seam allowance. Each selected top or bottom edge changes the height by one seam allowance. A piece with allowance on all four sides therefore gains two allowances in width and two in height.
Common Seam Allowance Sizes
These sizes are common references, not universal rules. A commercial garment pattern, quilting pattern, bag pattern or home-sewing project may specify something different.
Common quilting seam; also used for narrow construction seams.
Used by some garment, bag and independent sewing patterns.
A practical medium allowance for many sewing projects.
A long-standing common allowance in many US garment patterns.
Common metric allowance in garment and craft sewing.
Metric size close to 5/8 inch, though not exactly the same.
What Is a Standard Seam Allowance?
In US garment sewing, 5/8 inch has traditionally been common in many commercial patterns. Quilting commonly uses a 1/4-inch seam for piecing. Other patterns may use 3/8 inch, 1/2 inch, 1 cm, 1.5 cm or different allowances in different areas of the same project.
The pattern’s stated allowance is more important than any general convention. Some patterns include seam allowance in the printed pieces, while others require you to add it before cutting.
How to Add Seam Allowance to Finished Dimensions
Start with the finished size and identify the edges that will be stitched. Add one seam allowance for each selected edge. For example, a finished 10 × 6 inch rectangle with a 5/8-inch allowance on all four sides becomes an 11.25 × 7.25 inch cut piece.
If one edge is placed on a fold, that edge normally does not receive a seam allowance unless the pattern specifically says otherwise. Uncheck that edge in the calculator to model the piece correctly.
How to Remove Seam Allowance
To find finished dimensions from a cut piece, subtract the allowance from every edge where it is included. A 10-inch cut width with 1/2-inch seam allowances at both left and right finishes at 9 inches between those stitching lines.
Seam Allowance Inches to Centimeters
One inch equals exactly 2.54 centimeters. That makes 1/4 inch equal to 0.635 cm, 1/2 inch equal to 1.27 cm and 5/8 inch equal to 1.5875 cm. One centimeter is about 0.394 inch, while 1.5 cm is about 0.591 inch.
Measuring Seam Allowance Accurately
Seam allowance is measured from the fabric’s cut edge inward to the stitching line. Consistency matters: even a small repeated difference can change the finished dimensions of patchwork, fitted garments, cushions and other pieced projects.
A marked sewing-machine plate, seam gauge, ruler or removable fabric marker can help maintain a consistent distance. For curved seams, measure at several points because the cut edge and stitching line are not parallel in the same way as a straight seam.
Grading Seam Allowances
Grading means trimming layered seam allowances to different widths so the finished seam has less bulk. It is especially useful where several layers are enclosed. Grade only after the seam is sewn and avoid cutting too close to the stitching line.
Seam Allowance FAQ
Is seam allowance included in a sewing pattern?
Sometimes. Many patterns include it, but others do not. Check the pattern instructions or pattern-piece notes before cutting.
What is the most common seam allowance in quilting?
A 1/4-inch seam is common for quilt piecing because block dimensions are usually designed around that allowance.
Is 1.5 cm the same as 5/8 inch?
No. They are close but not identical. 1.5 cm is about 0.591 inch, while 5/8 inch is 0.625 inch or about 1.588 cm.
Where do I measure seam allowance from?
Measure from the cut edge of the fabric to the intended stitching line.
Do fold edges need seam allowance?
Usually a fold edge does not need seam allowance because it is not joined to another cut edge, but the pattern instructions control the final decision.
Can different edges have different seam allowances?
Yes. Garment necklines, hems, side seams and enclosed seams may use different amounts. The calculator handles one allowance value at a time, so calculate separate edge groups when a pattern mixes sizes.