Seam Allowance Calculator
Allowance is added at both opposing edges, so each direction gains twice the allowance. A 40 × 50 cm finished piece at 1 cm allowance is cut 42 × 52 cm. Add it once and the finished piece comes out 2 cm small.
Your numbers
Cut width
42cm
Cut length
52cm
Formula
Cut width = Finished width + 2 × Seam allowance, Cut length = Finished length + 2 × Seam allowance
The standard allowance differs by tradition: 1 cm in Korean and Japanese garment patterns, 5/8 in (1.6 cm) in American ones, 1/4 in (0.6 cm) in quilting. A double-folded hem needs twice the fold depth added on top.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Finished width (cm)The width when finished, before any seam or fold allowance. | 40 | 1 and up |
| Finished length (cm)The length when finished. | 50 | 1 and up |
| Seam allowance (cm)Allowance on one edge; it is added on both, so twice over. | 1 | 0 ~ 5 |
Step by step
Quick reference table
Results when only Finished width (cm) changes and everything else stays put.
| Finished width (cm) | Cut width (cm) | Cut length (cm) |
|---|---|---|
| 20 | 22 | 52 |
| 30 | 32 | 52 |
| 40 | 42 | 52 |
| 60 | 62 | 52 |
| 80 | 82 | 52 |
What each result means
| Result | At default values |
|---|---|
| Cut width (cm)The width to cut — finished size plus allowance. | 42 |
| Cut length (cm)The length to cut, finished length plus allowance. | 52 |
Common mistakes
The standard allowance differs by tradition: 1 cm in Korean and Japanese garment patterns, 5/8 in (1.6 cm) in American ones, 1/4 in (0.6 cm) in quilting. A double-folded hem needs twice the fold depth added on top.
Glossary
- Finished width
- The width when finished, before any seam or fold allowance.
- Finished length
- The length when finished.
- Seam allowance
- Allowance on one edge; it is added on both, so twice over.
- Cut width
- The width to cut — finished size plus allowance.
- Cut length
- The length to cut, finished length plus allowance.
Frequently asked questions
Q. How is Seam Allowance Calculator calculated?
Cut width = Finished width + 2 × Seam allowance, Cut length = Finished length + 2 × Seam allowance — Allowance is added at both opposing edges, so each direction gains twice the allowance. A 40 × 50 cm finished piece at 1 cm allowance is cut 42 × 52 cm. Add it once and the finished piece comes out 2 cm small.
Q. Can you walk through an example?
With Finished width 40cm, Finished length 50cm, Seam allowance 1cm, the answer is Cut width 42cm.
Q. What do I need to enter?
Enter Finished width, Finished length, Seam allowance. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Finished width (cm) moves the answer to Finished width (cm) 20 → Cut width (cm) 22 and Finished width (cm) 80 → Cut width (cm) 82. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
The standard allowance differs by tradition: 1 cm in Korean and Japanese garment patterns, 5/8 in (1.6 cm) in American ones, 1/4 in (0.6 cm) in quilting. A double-folded hem needs twice the fold depth added on top.
Related calculators
Materials differ in density and absorption. If your label states a figure, use it — the defaults here are only common values.