Work out seam allowance instantly with clear inputs, formula shown and shareable results.
Each dimension gains twice the seam allowance because there is a seam on both edges, so the cut size is the finished size plus two allowances. On small pieces the seam allowance dominates: a 5 cm finished square at 1.5 cm allowance needs an 8 cm cut, using more than twice the fabric area.
Cut size
cut dimension = finished dimension + 2 x seam allowance
cut dimension = finished dimension + 2 x seam allowance. Each dimension gains twice the seam allowance because there is a seam on both edges, so the cut size is the finished size plus two allowances.
On small pieces the seam allowance dominates: a 5 cm finished square at 1.5 cm allowance needs an 8 cm cut, using more than twice the fabric area.
Enter finished piece width, finished piece height, seam allowance per edge. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.