Checked reference
Common Seam and Hem Allowance Reference
The amounts below are examples, not universal settings. Construction, fabric, pattern, edge treatment, and regional practice vary. A tested commercial pattern instruction takes precedence over a general reference.
Reviewed on 2026-07-13.
| Item | Edge purpose | What to enter | Note |
|---|---|---|---|
| Straight seam | Chosen seam allowance outside the seam line | Use the pattern-specific amount. | |
| Narrow turned edge | Total extension required by all turns | Do not enter one fold twice unless the tool asks for each fold. | |
| Deep hem | Complete extension beyond the finished edge | Construction and fabric determine suitability. | |
| Zero allowance | 0 in the selected unit | Valid when the edge already represents a cut line. |
Variables and units
CW = finished width + left allowance + right allowanceCH = finished height + top allowance + bottom allowanceAll variables must use compatible length units.
Worked example
A 40-by-40-centimeter finished square with 1.5 centimeters on every edge becomes a 43-by-43-centimeter cut square.
Cautions
- Ease belongs in the finished measurement, not in seam allowance.
- Commercial pattern instructions take precedence.
Specific commercial pattern instructions take precedence whenever they differ from this general reference.