What is sewing seam allowance calculator?
A seam allowance calculator helps adjust simple pattern dimensions when the allowance changes. It is most reliable for rectangles and straightforward pieces where both opposite edges are affected equally.
How to use this calculator
- Enter the original seam allowance.
- Enter the target seam allowance.
- Enter pattern piece dimensions.
- Apply the edge change only to edges that actually have seams.
How is it calculated?
Formula
new dimension = original dimension + 2 x (target seam allowance - original seam allowance) What The Constants Mean
- Dimension adjustment (two opposite edges): Width and height adjustments assume seam allowance changes on both opposing edges.
Changing a 10 x 12 in rectangle from 1/4 in to 1/2 in seam allowance makes it 10.5 x 12.5 in.
Common mistakes
Do not add allowance to fold edges, finished edges, or edges already drafted without seams unless the pattern calls for it. Curves and complex garments may need pattern-walking rather than simple rectangle math.
FAQ
Can I use this for quilt blocks?
Yes, for simple block pieces when changing seam allowance consistently.
What about fold lines?
Do not double the change on a fold line because there is no seam on that edge.
Does this work for curves?
It can estimate, but curved pattern adjustments are better done with drafting tools.
Can I reduce seam allowance?
Yes. The change will be negative when the target allowance is smaller.