Seam Allowance Calculator
Add seam allowance to a finished pattern piece's dimensions to find the size you need to actually cut the fabric.
How the cut size is worked out
An allowance is added at each of the four edges, so each dimension grows by twice the allowance, not once:
cut width = finished width + (2 × seam allowance)
cut length = finished length + (2 × seam allowance)The defaults - a 10 × 14 in finished piece at 5/8 in - cut at 11.25 × 15.25 in. Adding a single 0.625 would give 10.625, and the piece would finish 5/8 in narrow in both directions. Results print to three decimals because metric allowances do not land on neat fractions: 1 cm entered as 0.394 in makes a cut width of 10.788 in, near enough 10 13/16 in.
Which allowance to use
| Allowance | Decimal | Used for |
|---|---|---|
| 1/4 in | 0.25 | Quilt piecing |
| 3/8 in | 0.375 | Knits, serged seams |
| 1/2 in | 0.5 | Bag and craft patterns |
| 5/8 in | 0.625 | Commercial garment patterns |
A 12 in finished quilt block at 1/4 in cuts at 12.5 in, which is where the familiar half inch on block sizes comes from.
Frequently asked questions
What is the standard seam allowance in sewing?
5/8 in on commercial garment patterns, 1/4 in for quilt piecing and 1 cm on most European ones. The pattern envelope states it; do not assume.
Do I add seam allowance to both sides of a piece?
Yes. Both edges of the width and both of the length, which is why the tool adds twice the allowance. A 10 in finished width at 5/8 in cuts at 11.25 in.
What is 5/8 inch seam allowance in cm?
1.59 cm. Patterns drafted in metric normally use a round 1 cm, so a piece swapped between the two systems finishes about 1.2 cm wider or narrower.