Seam allowance sizes by project type
Why quilters use 1/4 inch, garments use 5/8 inch, and bags use 1/2 inch, plus how each choice changes the size you cut.
Why the allowance sits on every edge
A seam allowance is the strip of fabric between the cut edge and the stitched seam line. When two pieces are joined, the machine consumes that strip on both pieces, so the visible finished piece is smaller than what you cut. Because a rectangle has an allowance on each of its two opposing edges, every dimension shrinks by two allowances once sewn. That is why the cut size must add twice the allowance to each dimension, and why a generous allowance can quietly swallow an inch or more if you forget it.
Quilting at a quarter inch
Quilters standardize on a 1/4 inch allowance because blocks are pieced from many small units, and a narrow, consistent allowance keeps points sharp and seams flat under the quilting. A 6 inch finished block therefore cuts at 6.5 inches square, adding a quarter inch to each of the four edges. Many quilters chase a scant quarter inch, a thread or two under 1/4 inch, to absorb the fabric that a pressed seam takes up. If your finished blocks come out slightly small, test a scant allowance rather than changing the cut size.
Garments at five eighths
Commercial dressmaking patterns almost always use a 5/8 inch allowance, which gives room to let a garment out, to finish raw edges, and to sew curved seams cleanly. A 20 inch finished panel cuts at 21.25 inches wide once you add 5/8 inch to each side. The wider allowance is also forgiving on fitting muslins, since you can restitch a seam a little narrower to add ease. Serged or knit garments sometimes drop to 3/8 inch, so always read the pattern envelope before cutting.
Bags, home decor and the half inch habit
Bags, pillows, cushions and many home projects settle on a 1/2 inch allowance as a sturdy middle ground. It is wide enough to hold up to heavy interfacing, zippers and topstitching, yet not so wide that it adds bulk in the corners. A 16 inch finished pillow front cuts at 17 inches square with a half inch on each edge. When a pattern does not state an allowance, a half inch is a safe default for structured projects, while a quarter inch suits delicate or pieced work.