Seam Allowance Converter
Convert seam allowances between inch fractions and millimeters, and see total added measurement for pattern pieces.
| Brand / Origin | Standard S.A. | In mm | Notes |
|---|---|---|---|
| Simplicity | 5/8" | 15.9 mm | US standard; included in pattern pieces |
| McCall's / Butterick / Vogue | 5/8" | 15.9 mm | US standard; printed on pattern |
| Burda Style | 1.5 cm | 15 mm | European; S.A. NOT included — must add when cutting |
| Knip Mode / Dutch patterns | 1 cm | 10 mm | S.A. NOT included on pattern pieces |
| Japanese patterns | 1 cm | 10 mm | S.A. sometimes NOT included; check instructions |
| Indie / PDF patterns | Varies | — | Always check pattern cover page for S.A. specification |
| Quilting patterns | 1/4" | 6.4 mm | Precise 1/4" is critical for block accuracy |
What Is a Seam Allowance?
A seam allowance is the extra fabric between the stitch line and the cut edge of the fabric. It provides fabric to hold the seam, prevent fraying, and allows for adjustments. Most commercial patterns from the US include a 5/8" (15.9 mm) seam allowance. European patterns like Burda print the cut line without seam allowance, so you must add it when cutting.
Why Both Sides Matter
When two pieces are joined at a seam, each piece carries one seam allowance. That means the total fabric consumed by a seam is the seam allowance width multiplied by two. When resizing a pattern piece or calculating fabric yardage, you must account for both sides. For example, a garment with a 5/8" seam allowance uses 1-1/4" of total fabric width at each seam — not just 5/8".
Converting Between Pattern Systems
If you are adapting a Burda pattern (1.5 cm allowance) to a Simplicity standard (5/8" = 15.9 mm), the difference is only 0.9 mm — essentially the same. But converting a Japanese 1 cm pattern to a US 5/8" pattern requires adding an extra 5.9 mm to every cut edge. Always mark the stitch line on your fabric and measure from there, regardless of the seam allowance width.