Seam Allowance Converter
Convert seam allowances between millimeters and inches.
Get fraction equivalents and fabric-specific advice for perfect garment construction.
What Is a Seam Allowance?
A seam allowance is the fabric between the cut edge and the stitching line. It is the extra material that gets folded inside the garment when two pieces are sewn together. Getting the seam allowance right is one of the most important factors in achieving a well-fitting, professional-looking garment.
Standard Seam Allowances by Country
Seam allowances vary by country and tradition. In the United States, most commercial patterns use 5/8 inch (15.9 mm). In the United Kingdom and Australia, 1.5 cm (15 mm) is the standard. In continental Europe, patterns often use 1 cm (10 mm). Knowing which standard your pattern follows prevents costly fitting mistakes.
Seam Allowances by Fabric Type
The correct seam allowance depends on the fabric you are using. Sheer or delicate fabrics like chiffon and organza do best with narrow seam allowances of 6–10 mm (1/4 inch). This prevents bulk from showing through the fabric. Woven cottons and linens work well with 12–16 mm (1/2 to 5/8 inch) seam allowances — enough to press open neatly and handle multiple wash cycles. Heavy fabrics like denim, canvas, and upholstery fabric need at least 16–25 mm (5/8 to 1 inch) to hold securely under stress. Knit fabrics are typically sewn with a 6 mm (1/4 inch) allowance using a serger or zigzag stitch, since the seam must stretch with the fabric.
Pressed vs. Trimmed Allowances
After sewing, seam allowances are either pressed open (each side folded away from the seam line) or pressed to one side. For curved seams such as necklines and armholes, you must clip or notch the allowance so it lies flat. Grading (trimming each layer to a different width) reduces bulk in areas like collars and cuffs. Understanding how much allowance you have lets you grade and trim with confidence.
Why Precision Matters
Even a 2 mm difference in seam allowance across multiple seams can add up to a visible fit problem in the finished garment. Always measure twice and mark clearly with tailor’s chalk or a seam gauge before cutting.