Seam Allowance Calculator — Add or Trim Seam Allowance
Calculate seam allowance adjustments when converting between different seam allowance standards.
1.5cm (5/8″) is the UK standard
New SA you want to use
Current piece length (inc. SA)
Current piece width (inc. SA)
Edges with seam allowance (4 for a rectangle)
How We Calculate This
To change seam allowance, add or subtract the difference from each edge of the pattern piece.
The formula
Difference per edge = Desired SA − Current SA
Add this amount to each seam edge (positive = enlarge, negative = trim).
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates. Always verify quantities before cutting or purchasing fabric.