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Seam allowance converter
Inch fractions to mm and back, with the standards.
In short
The quilting seam allowance is 1/4 in (6.35 mm, or 0.75 cm in metric patterns). Garment patterns from the US Big 4, the UK and Europe use 5/8 in (1.5 cm). Overlocker patterns for knits use 1/4 to 3/8 in (6 mm to 1 cm).
From
equals
Centimetres
Inches
Nearest 1/8 in
Seam allowance standards
| Imperial | Metric | Used for |
|---|---|---|
| 1/4 in | 6.35 mm (0.75 cm) | Quilting and patchwork |
| 1/4 to 3/8 in | 6 mm to 1 cm | Knits on an overlocker |
| 3/8 in | 1 cm | Many indie patterns |
| 1/2 in | 1.3 cm | Bags, home decor and some indie patterns |
| 5/8 in | 1.5 cm | US Big 4, UK and EU commercial patterns |
| 3/4 to 1 1/2 in | 2 to 4 cm | Hems on many garments |
How to use it
- Type an allowance, or tap a common size.
- Switch between inches and centimetres.
- Use the nearest 1/8 in mark on your ruler or seam guide.
Pip's tips
- Always read your pattern first: Burda magazine, OTTOBRE and most Japanese pattern books leave allowances for you to add.
- Test your quarter inch: three 1 1/2 in strips sewn together should measure exactly 3 1/2 in.
- A stack of masking tape or a magnetic seam guide keeps every seam spot on.
Sources: Every number follows the SewNQuilt quilting and sewing standards, built from published industry references. Imperial and metric are calculated natively, never converted piece by piece.
Keep this with you
Save every result to a project, add fabric to your shopping list and work offline in the SewNQuilt app.
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