Seam Allowance Calculator
Add a seam allowance to a finished measurement to get the cut size, and work back from a cut size to the finished size it produces.
At a glance
- What you enter
- Finished width, Finished length, Seam allowance, How many edges take the allowance, Cut width you already have
- What you get
- Cut width to mark and cut, Cut length to mark and cut, Finished width, Share lost to seams, Difference from your target
- Formula shown
- Yes, every step, right below your results.
- Cost and privacy
- Free, no account needed, and nothing you type is stored.
See worked examples ↓ Read the FAQ ↓ Share this calculator ↓
Results are estimates for general information only, always verify with a professional or official source before acting on them.
How to use the Seam Allowance Calculator
- 1 Enter Finished width (in), What you want the piece to measure once it is sewn.
- 2 Enter Finished length (in)
- 3 Enter Seam allowance (in), 5/8 in on a commercial pattern, 1/4 in for quilt piecing, 3/8 in for an overlocked seam.
- 4 Enter How many edges take the allowance
- 5 Enter Cut width you already have (in), Fill this in to work backwards from a cut size. Leave at zero to use the finished width above.
- 6 Press Calculate. You get Cut width to mark and cut, Cut length to mark and cut, Finished width, Share lost to seams, Difference from your target, and every intermediate step stays visible under "How is this calculated?"
- 7 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.
Worked examples
Real numbers run through this calculator. Open any example to load it into the form above and change it.
A bodice panel with two side seams
A 20 inch finished width at 5/8 inch seam allowance, with a seam at each side.
You enter
- Finished width
- 20 in
- Finished length
- 30 in
- Seam allowance
- 0.63 in
- How many edges take the allowance
- 2
- Cut width you already have
- 0 in
You get
- Cut width to mark and cut
- 21.25 in
- Cut length to mark and cut
- 31.25 in
- Finished width
- 20.00 in
- Share lost to seams
- 9.6%
- Difference from your target
- 0.0%
Cut 21 1/4 inches wide, not 20 5/8, because the allowance applies at both edges.
Working back from a cut panel
You have a 21.5 inch wide panel and want to know what it finishes at with a 5/8 inch seam.
You enter
- Finished width
- 20 in
- Finished length
- 30 in
- Seam allowance
- 0.63 in
- How many edges take the allowance
- 2
- Cut width you already have
- 21.50 in
You get
- Cut width to mark and cut
- 21.50 in
- Cut length to mark and cut
- 31.25 in
- Finished width
- 20.25 in
- Share lost to seams
- 9.6%
- Difference from your target
- 1.3%
Two seams take 1 1/4 inches, so the finished width is 20 1/4 inches.
Guides that go with it
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Frequently asked questions
How do I add seam allowance to a pattern piece?
What seam allowance do commercial patterns use?
Does seam allowance affect the finished size of a quilt?
How do I work out how much to let out a seam?
Terms used here
Short, plain-language definitions of the words this calculator uses.