Why a quarter-inch seam matters
Patchwork commonly uses a ¼-inch seam allowance. Fabric inside the seam is not visible in the finished top, so a piece specified by finished size needs allowance on every edge that will be sewn. For a basic rectangle, add twice the seam allowance to each dimension.
With ¼-inch seams, a 4-inch finished square is cut at 4½ inches. A border that finishes 3 inches wide is cut 3½ inches wide. Sashing that finishes 2 inches wide is cut 2½ inches wide. The same rule applies in metric when the chosen allowance is entered in millimetres or centimetres.
Nominal, scant, and tested seams
Thread, pressing, and the fold of the fabric can make a sewn unit finish a little small even when the needle is nominally ¼ inch from the edge. Some quilters compensate with a scant ¼-inch seam. QuiltClarity does not silently choose that adjustment: enter the allowance your tested construction requires.
- Cut a small test set using the planned dimensions.
- Sew and press it with the project’s thread and pressing method.
- Measure the unit before repeating the cut for the whole quilt.
Where to apply it
Add allowance when converting a finished design measurement into a cut measurement. Do not add it again to cutting dimensions supplied by a pattern. The finished-vs-cut guide shows that distinction. For project-specific math, use the border calculator or sashing calculator; both expose the seam allowance they use.