Sewing reference & tool
Seam allowance chart and calculator
Seam allowance is the distance between the stitch line and the fabric’s raw edge. Use the value printed in your pattern; there is no single allowance that applies to every pattern, seam, or edge.
Exact unit conversion
Common seam allowance conversions
The metric values use the exact relationship 1 inch = 25.4 millimeters. Decimal values are shown precisely; a sewing guide or machine plate may use a practical rounded mark.
| Inches | Millimeters | Centimeters | Where you may see it |
|---|---|---|---|
| 1/8 in | 3.175 mm | 0.318 cm | Narrow finishing and precision work when specified |
| 1/4 in | 6.35 mm | 0.635 cm | Common in patchwork and some enclosed seams |
| 3/8 in | 9.525 mm | 0.953 cm | Used by some knit and independent patterns |
| 1/2 in | 12.7 mm | 1.27 cm | General construction when the pattern specifies it |
| 5/8 in | 15.875 mm | 1.588 cm | Common on many U.S. garment patterns |
| 3/4 in | 19.05 mm | 1.905 cm | Wider allowance for selected construction details |
| 1 in | 25.4 mm | 2.54 cm | Often an alteration, fitting, or hem allowance rather than a default seam |
Add or remove seam allowance
For rectangular dimensions with the same allowance on all four edges.
Calculated rectangle
This rectangle math does not offset curves, corners, darts, hems, facings, or asymmetric edges.
Use it correctly
What the calculator adds
For a rectangle, adding the same allowance to every edge increases both width and height by twice the allowance. An 8 × 10 inch seam-line rectangle with a 5/8 inch allowance becomes a 9.25 × 11.25 inch cut rectangle.
Removing allowance reverses that calculation. It is useful for checking a simple rectangular piece, but it is not a pattern-drafting tool: a true offset around curves and angled corners must follow the shape of the seam line.
Hems, fold lines, facings, binding, flat-felled seams, French seams, and graded or trimmed seams can use different amounts on different edges. Keep those pattern markings intact.
At the machine
Measure from the raw edge to the needle.
Align the raw edge with the correct machine guide and keep the edge parallel as you sew. Confirm the mark with a ruler if the plate is unfamiliar, especially when working from a converted metric value.
Source notes
Last reviewed September 13, 2026
- Seamwork: How to Sew a Basic Seam Definition of seam allowance and pattern-first guidance for choosing it.
- National Institute of Standards and Technology: SI Units — Length Exact inch-to-millimeter relationship: 1 inch equals 25.4 millimeters.