Sewing Seam Allowance Calculator
Cut a piece at its finished size and the seams will eat into it, leaving the garment too small. Seam allowance fixes that by adding extra fabric on each edge to be sewn. Enter the finished width and height you want, the seam allowance per edge, and this calculator returns the cut width and height (allowance added on both opposite edges), the area of fabric needed, and the finished size implied by a cut size. Keep all values in the same unit, inches or centimeters. The math is plain addition, so the result is exact.
Seam allowance formula
Added per dimension = 2 * seam allowance
Cut width = finished width + 2 * seam allowance
Cut height = finished height + 2 * seam allowance
Cut area = cut width * cut height
Each dimension gains twice the allowance because it is seamed on two opposite edges. If one edge is a fold or already finished, add the allowance only once for that dimension.
Common allowances
- Quilting commonly uses a 0.25 inch (6 mm) seam allowance.
- US garment patterns often use 0.625 inch, written as 5/8 inch.
- Knits and sergers may use a narrower allowance such as 0.375 inch.
- Always defer to the allowance printed on your pattern.
- Keep finished size and allowance in the same unit, inches or centimeters.
Seam allowance: frequently asked questions
How do I add seam allowance to a pattern?
Seam allowance is added on every edge that will be sewn. For a piece sewn on both opposite edges, the cut dimension equals the finished dimension plus twice the seam allowance. A 10 inch finished width with a 0.5 inch allowance on each side is cut at 11 inches.
What is a standard seam allowance?
Common allowances are 0.25 inch (6 mm) for quilting and 0.625 inch (1.6 cm), often written as 5/8 inch, for garment sewing in the US. Always follow your pattern's stated allowance; enter it here as a decimal and the calculator does the rest.
Why multiply the allowance by two?
Because a typical piece is seamed on two opposite edges, allowance is added at both. If only one edge is seamed (the other is a fold or finished edge), add the allowance once instead. The calculator shows both the per-edge and both-edges results.
How do I convert a cut size back to finished?
Subtract twice the seam allowance from the cut dimension. This calculator reports the finished size implied by your cut size and allowance so you can check a piece either way.
Does this handle metric measurements?
Yes. Work entirely in inches or entirely in centimeters; just keep your finished dimension and allowance in the same unit. The arithmetic is identical and the calculator does not assume a unit.
Official sources
- National Institute of Standards and Technology: inch and centimeter definitions.
- U.S. Department of Agriculture Extension: home sewing education resources.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.