Start from the stitching line
The finished boundary is normally the stitching line. A rectangle finished at 20 × 30 cm with a 1 cm allowance on every side is cut at 22 × 32 cm.
- Add the allowance to both opposing sides.
- For a circle, add it to the radius—not just once to the diameter.
- For a circle-skirt waist, the cutting radius moves inward because the seam allowance sits above the waist stitching line.
Avoid double-counting
Some commercial patterns already include seam allowance. If you enter a pattern piece’s complete cut dimensions, set additional seam allowance to zero. Add it in the calculator only when your inputs describe finished dimensions.
- Check the pattern legend.
- Mark stitching and cutting lines differently.
- Keep hem allowance separate from seam allowance.
Choose an allowance for the construction
The correct allowance depends on fabric stability, seam finish, fitting needs and technique. A narrow enclosed seam and a fitting seam that may be altered do not have the same needs.
- Use the pattern or method specification when one exists.
- Test bulky or fraying fabrics.
- Do not trim allowances until the relevant fit and construction checks are complete.
Why the diagram matters
A trustworthy layout must draw the complete cut shape inside the usable fabric boundary. That is why the Studio performs geometry with seam allowance included before it counts rows or recommends a purchase.