Seam allowances
The seam allowance is the fabric between the stitching line and the cut edge. Forget it and every piece comes out too small. Add it twice and you'll buy too much.
Common sizes (always follow your pattern)
| Use | Typical allowance |
|---|---|
| US commercial garment patterns | 5/8 in (about 1.5 cm) |
| Quilting and patchwork | 1/4 in (about 6–7 mm) |
| Many indie and European patterns | 1 cm or 1/2 in, and some are sold without allowances |
| Home décor (cushions, curtains) | often 1/2 in or 1.5 cm; hems are separate |
These are conventions, not rules. Allowances vary by brand and even by seam (Threads magazine, "All About Sewing Industry Seam Allowances").
Included or separate?
Read the pattern's key. In the planner, tick Seam allowance already included when your measurements are already cut sizes. Otherwise the planner adds the allowance to all four sides. Hems, folds and edges that aren't sewn usually need different amounts, so enter cut sizes directly for those pieces.