A clear unit check
Seam Allowance Converter
Translate a measurement from your pattern into millimetres, centimetres or inches. Keep the original instruction beside your result. This tool changes the unit, not the allowance your project needs.
Convert a measurement
Enter a positive decimal number. For fractional inches, use the decimal equivalents in the table.
Illustrative conversion
- Millimetres
- mm
- Centimetres
- cm
- Inches
- in
The initial 5/8 inch entry is an illustrative conversion, not a recommended seam allowance.
Displayed numbers are rounded for readability. Keep your pattern's original instruction as the source.
Inch fractions
The same length,
another unit.
One inch is exactly 25.4 millimetres. One centimetre is 10 millimetres.
| Fraction | Decimal in | mm | cm |
|---|---|---|---|
| 1/4 in | 0.25 | 6.35 | 0.635 |
| 3/8 in | 0.375 | 9.525 | 0.9525 |
| 1/2 in | 0.5 | 12.7 | 1.27 |
| 5/8 in | 0.625 | 15.875 | 1.5875 |
Keep the instruction in view
A conversion is one small check.
Seam allowance and stitch length
Seam allowance is the width between a seam's stitching line and the fabric cut edge. It may be included in a pattern piece, added separately, or vary across a project. Read the instruction for that particular seam or edge.
Stitch length is different: it describes the distance between successive machine stitches. A stitch length and an allowance can share the same unit while describing different things. This converter handles lengths; it does not choose a stitch setting or inspect a sewn seam.
Use the converted number as a note beside the source measurement. The table gives equivalents, not preferred widths. A familiar fraction is not a reason to replace a different pattern instruction.
Printing, sizing and materials
Pattern scaling changes many distances at once. Converting a stated allowance is not a reason to resize a PDF. Follow the printing and assembly guide and check the pattern's supplied print reference in both directions.
An allowance also does not select a pattern size or determine garment ease. Use the pattern's measurement chart and Sewlore's size guidance for that preparation step. Unit arithmetic cannot grade a pattern or assess its fit.
Material handling, shrinkage, stretch and seam bulk need observations from your own fabric and sample work. Read the fabric and elastic preparation guide when considering those requirements. The converter reports only the length entered.