The Challenge
American sewing patterns use inches for seam allowances. European and Japanese patterns use centimeters. The standard US seam allowance of 5/8 inch is 1.587cm — not a round number, which is why mixing systems mid-project causes fit problems. Burda patterns use 1.5cm (0.591in). Big Four US patterns (Simplicity, McCall's, Butterick, Vogue) default to 5/8in (1.587cm). Japanese sewing books typically use 1cm seam allowances. None of these are interchangeable — a 1mm difference in seam allowance across 20 seams shifts a garment's finished size by 4cm.
Seam Allowance Conversion Chart: Inches to Centimeters
| Inches (fraction) | Inches (decimal) | Centimeters | Common Use |
|---|---|---|---|
| 1/8 in | 0.125 in | 0.318 cm | Edgestitching, fine details |
| 3/16 in | 0.1875 in | 0.476 cm | Lingerie, lightweight knits |
| 1/4 in | 0.250 in | 0.635 cm | Quilting standard |
| 3/8 in | 0.375 in | 0.953 cm | Curved seams, children's wear |
| 1/2 in | 0.500 in | 1.270 cm | Knit fabrics, some home dec |
| 5/8 in | 0.625 in | 1.587 cm | US Big Four garment standard |
| 3/4 in | 0.750 in | 1.905 cm | Outerwear, heavy fabrics |
| 1 in | 1.000 in | 2.540 cm | Waistbands, hems, alterations |
| 1 1/4 in | 1.250 in | 3.175 cm | Wide hem allowance |
| 1 1/2 in | 1.500 in | 3.810 cm | Deep hem, trouser turn-up |
| 2 in | 2.000 in | 5.080 cm | Hem allowance, wide seams |
Seam Allowances by Pattern Brand and Country
| Pattern Brand / Origin | Standard Seam Allowance | Included in Pattern? | Notes |
|---|---|---|---|
| Simplicity (US) | 5/8 in (1.587 cm) | Yes | Printed on cutting lines |
| McCall's (US) | 5/8 in (1.587 cm) | Yes | Printed on cutting lines |
| Butterick (US) | 5/8 in (1.587 cm) | Yes | Printed on cutting lines |
| Vogue (US) | 5/8 in (1.587 cm) | Yes | Printed on cutting lines |
| Burda (Germany) | 1.5 cm (0.591 in) | No | Must be added when cutting |
| Ottobre (Finland) | 1.0 cm (0.394 in) | Yes | Children's: 1cm, adults: 1.5cm |
| Named (Norway) | 1.0–1.5 cm | Yes | Stated per pattern |
| Closet Core (Canada) | 5/8 in (1.587 cm) | Yes | US standard |
| Japanese indie patterns | 1.0 cm (0.394 in) | Yes | Usually included |
| Seamwork (US) | 1/2 in (1.270 cm) | Yes | Beginner-friendly smaller SA |
| Colette (US) | 5/8 in (1.587 cm) | Yes | US standard |
Why 5/8 Inch Became the US Garment Standard
The 5/8 inch (1.587cm) seam allowance became standard in the US commercial pattern industry in the 1950s when Simplicity, McCall's, and Butterick standardized their printing. The measurement was chosen as a practical balance: wide enough to prevent fraying on woven fabrics without a serger, narrow enough to reduce bulk in curved seams. Before standardization, patterns were printed without seam allowances and sewers added their own — the same approach Burda still uses today. The 5/8in standard persists because the entire US garment industry, from home sewing to RTW grading, is built around it. Switching to metric would require recalibrating every seam guide, presser foot marking, and cutting template in the workflow.
How to Adapt a US Pattern to Metric Seam Allowances
- Identify the seam allowance stated on the US pattern — usually 5/8in (1.587cm)
- Decide your target metric standard: 1.5cm is closest to 5/8in with only 0.087cm difference
- Redraw all cutting lines: shift each edge inward by the difference (0.087cm) or simply sew at 1.5cm and accept the minimal variance
- Mark your new seam allowance clearly on every pattern piece before cutting
- Update any notch positions — notches placed at 5/8in from edge must move to 1.5cm from edge
The Burda Pattern Trap
- Burda patterns print the stitching line only — no seam allowance is included in the printed pattern piece
- You must add 1.5cm to every edge when tracing or cutting — skipping this step makes every finished piece 3cm too small in each direction
- Burda instructions state this on page one but it is the single most common mistake when switching from US patterns
- US patterns print the cutting line (seam allowance already added) — the stitching line is the inner marked line
Inches vs Centimeters for Seam Allowances
Step-by-Step Workflow
Enter your seam allowance in inches — use decimals (0.625) or fractions noted on the pattern
Centimeter value appears instantly — compare against your pattern's printed marks
Use the chart below to verify standard allowances before cutting
Specifications
- Formula
- cm = inches × 2.54
- 5/8 in (US standard)
- 1.587 cm
- 1/2 in
- 1.270 cm
- 3/8 in
- 0.953 cm
- 1/4 in (quilting standard)
- 0.635 cm
- 1 cm (Japanese standard)
- 0.394 in
- 1.5 cm (Burda/EU standard)
- 0.591 in
Best Practices
- 5/8in = 1.587cm, not 1.5cm — the 0.087cm gap adds up across multiple seams
- Quilting uses 1/4in (0.635cm) seam allowances; garment sewing typically uses 5/8in (1.587cm)
- Burda patterns are printed without seam allowance — you add 1.5cm when cutting
- Japanese patterns often include seam allowance already — check the instruction sheet before adding more
- When scaling patterns between systems, change all allowances consistently — never mix inches and cm on the same piece
Frequently Asked Questions
What is 5/8 inch seam allowance in centimeters?
5/8 inch = 0.625 inches × 2.54 = 1.5875cm, rounded to 1.587cm. This is the US Big Four standard (Simplicity, McCall's, Butterick, Vogue). It is not the same as 1.5cm — the 0.0875cm difference per seam accumulates. On a fitted bodice with 12 seams, using 1.5cm instead of 1.587cm results in the finished garment being about 2cm larger than intended.
What is the standard seam allowance in centimeters for European patterns?
Burda (Germany) and most European commercial patterns use 1.5cm as the standard seam allowance. This equals 0.591 inches — slightly less than the US 5/8in standard. Burda patterns are unique in that they are printed without any seam allowance included; you must add 1.5cm around every piece when cutting.
What is 1/4 inch seam allowance in cm?
1/4 inch = 0.25 × 2.54 = 0.635cm. This is the quilting standard in the US. Most quilt patterns assume a scant 1/4in (slightly under 0.635cm) to account for thread thickness. In metric quilting patterns, the equivalent is 6mm or 0.6cm.
How do I convert a pattern from inches to centimeters accurately?
Multiply every measurement by 2.54. Do not round seam allowances to the nearest 0.5cm — a 5/8in seam is 1.587cm, not 1.5cm. For pattern pieces, convert the cutting lines and grain lines. For seam allowances specifically, decide whether to convert exactly or shift to a metric standard (1cm or 1.5cm) and apply consistently across all pieces.
What seam allowance do Japanese sewing patterns use?
Most Japanese home sewing patterns use 1cm seam allowances, with some using 1.5cm for heavier fabrics or stress seams. Unlike Burda, Japanese patterns typically include the seam allowance in the printed pattern pieces. Always read the pattern instructions — the seam allowance will be stated on the first or second page.
Why does seam allowance precision matter so much?
Garment fitting is cumulative. A side seam sewn 2mm wider than intended removes 2mm from each side — 4mm total per horizontal seam. A fitted dress with 8 cross-body seam lines sewn 2mm off loses 3.2cm from the finished width. That shifts a size 12 toward a size 10 fit. Precision within 1mm is the working standard for tailored garments.
What is 3/8 inch seam allowance in cm?
3/8 inch = 0.375 × 2.54 = 0.9525cm, effectively 0.95cm. This allowance appears on curved seams in some US patterns and on children's patterns where bulk reduction matters. It sits between the quilting 1/4in and the garment 1/2in standard.