Seam Allowance Calculator

How it is calculated

You enter the finished width, the finished height and the seam allowance per edge. The tool adds the allowance on both opposite edges of each dimension: cut width = finished width + 2 × allowance and cut height = finished height + 2 × allowance. The same unit applies to all three inputs and to the results (centimetres in, centimetres out; inches in, inches out); the tool does no conversion. Values are rounded to two decimals, and negative numbers give no result. The calculation assumes one uniform allowance on all four sides of a plain rectangle. It does not add extra for hems, folds, pleats, shrinkage or curved edges, which need their own allowances.

Common mistakes

• Adding the allowance only once per dimension. It is needed on both opposite edges, so it is counted twice.
• Mixing units, for example a finished size in inches with an allowance of 1.5 meant as centimetres. Keep one unit throughout.
• Using a garment allowance (such as 5/8 inch) for a quilt block, or the reverse. Different crafts use different allowances, and the pattern's value should be used.
• Entering the cut size as the finished size. The finished size is what you measure after sewing, inside the seam lines.
• Forgetting extra for hems, shrinkage or fabric grain. This tool covers seam allowance only.