Seam Allowance Calculator
Add or remove seam allowance to convert between a finished measurement and a cut measurement.
Reviewed by the ToolNestr Editorial Team — July 2026
How seam allowance calculation works
Seam allowance is the extra fabric added beyond a pattern piece's finished size so there's something to actually sew when two pieces are joined together — without it, stitching two exact-finished-size pieces together would produce a garment smaller than intended. This calculator adds your seam allowance amount once for each seamed edge on a dimension: if both the left and right edges of the width are seamed, the full seam allowance is added twice (once per edge) to that width.
A folded edge (like a piece cut on the fabric fold, common for center-front or center-back pieces) has no seam there, so setting that dimension's edge count to zero leaves it at the finished measurement with no addition.
Five-eighths inch (1.6 cm) is the standard default seam allowance for major US commercial pattern companies; always confirm the specific value stated on your pattern.
Worked example
A finished 20 × 30 inch panel, cut on the fold along one 30-inch edge, with 5/8-inch seam allowance elsewhere.
Real-world context
Drafting an original pattern from scratch, rather than working from a printed commercial pattern, is where this calculation happens constantly — every piece starts as a finished-size shape and needs seam allowance added on paper before it can be cut from fabric, and forgetting even one edge results in a piece that comes out smaller than intended once sewn.
Converting a pattern between two different seam allowance conventions (a European pattern using 1cm against a US pattern using 5/8 inch) also relies on this same add-and-remove logic — subtracting the original allowance to get back to finished size, then adding the new desired allowance.
Common misconceptions
"Seam allowance is added once per pattern piece, not per edge." Not quite. It's added to every seamed edge individually — a piece seamed on all four sides gets the allowance added twice to each dimension (once per opposing edge), not once total.
"All commercial patterns use the same seam allowance." They don't. While 5/8 inch is common in the US, many independent and international patterns use 3/8 inch, 1cm, or even no seam allowance at all (finished-size-only patterns) — always check before cutting.
Related tools
Frequently asked questions
What seam allowance should I use?
Five-eighths of an inch (1.6 cm) is the standard commercial pattern default in the US, while many independent patterns use three-eighths (1 cm) or a quarter inch. Always check the specific pattern's stated seam allowance rather than assuming.
Do all edges of a pattern piece get seam allowance?
Not necessarily — a fold line (like the center front of a piece cut on the fold) does not get seam allowance, so this calculator lets you set how many edges of each dimension are actually seamed.
Why would I need to remove seam allowance instead of add it?
If you have a cut pattern piece (from a garment you're copying, or a draft with seam allowance already built in) and want to know the finished, sewn size, you subtract the seam allowance instead of adding it.
Does hem allowance work the same way as seam allowance?
The same add/subtract logic applies, but hem allowances are typically larger (1-2 inches) than seam allowances, and only apply to the bottom edge rather than every seamed edge — calculate it separately using the hem edge count set to 1.