How to calculate it
Cut width = finished width + twice the seam allowance. Cut length = finished length + twice the seam allowance. The allowance is applied to both edges of each dimension.
Worked example
A finished 12 × 18 in panel with ½ in seams on all sides is cut 13 × 19 in.
Before you cut or buy
- This assumes a straight rectangular panel and seams on all four sides.
- Hems, facings, fold lines and turn-of-cloth need separate allowances from the pattern.
Common questions
Why is the allowance added twice to each measurement?
A rectangular piece has two opposite edges for each dimension. A half-inch seam on the left and right adds one inch to the cut width; top and bottom add one inch to the cut length.
Does the cut size include a hem?
Only if the entered per-edge allowance is appropriate for that hem. Most hems differ from joining seams, so calculate those edges separately using the pattern instructions.
Use your own measured materials and pattern instructions. Estimates cannot account for every variation in technique, shrinkage or layout.