Seam Allowance Calculator
Cut size from finished size, and back again.
Calculate cut size from a finished size and seam allowance, or work backwards from a cut piece, in inches or centimetres.
Cut size
10.50"
0.25" allowance on 2 edges
Seam allowance drift is the usual reason a finished garment comes out small: a sixteenth of an inch extra on each of eight seams is half an inch across the piece, which is most of a size. Set the machine guide once and check it against a scrap.
How the Seam Allowance Calculator works
Every edge that gets sewn loses its seam allowance to the seam, so a piece has to be cut larger than it will finish by the allowance on each sewn edge — twice the allowance for a piece sewn on both sides. It sounds trivial and it is the most common reason a first garment comes out small, because the error compounds across every seam in the piece.
Also known as: cut size calculator · sewing allowance calculator
How the number is derived
Each sewn edge loses its seam allowance from the finished dimension. A piece sewn on two opposite edges therefore finishes two allowances smaller than it was cut, so the cut size is the finished size plus twice the allowance.
That is the whole arithmetic, and it is trivial. What makes it worth a calculator is that the allowance varies by tradition: five eighths of an inch on commercial garment patterns, a quarter inch in quilting, three eighths on overlocked knits, and half an inch on many independent patterns.
Working in centimetres, the equivalents are 1.5 cm, 0.6 cm and 1 cm. Mixing systems mid-project is a reliable way to produce pieces that do not match.
A worked example
A 10 inch finished quilt block sewn on all four sides at a quarter inch is cut at 10.5 inches — a quarter added at each of the two edges in each dimension.
The same block at a commercial five eighths allowance would be cut at 11.25 inches. That inch and a quarter of difference across a block, multiplied by the number of blocks, is why a pattern's stated allowance is not a suggestion.
Where the answer misleads
The largest source of error is not the arithmetic but the sewing. A seam allowance a sixteenth of an inch wider than intended, repeated across eight seams, takes half an inch out of a garment — most of a size. Set the machine's guide once against a marked scrap rather than trusting the needle plate markings.
Curved seams behave differently from straight ones. On a tight curve the effective allowance changes as the seam turns, which is why sleeve heads and necklines are often sewn at a narrower allowance than the rest of a garment.
Finally, decide once whether your pattern includes allowance or not. Many drafting methods produce a net pattern and add allowance at the cutting stage; commercial patterns include it. Assuming the wrong one is a full allowance out on every edge.
Frequently asked questions
What is the standard seam allowance?
Five eighths of an inch, or 1.5 cm, on commercial garment patterns. Quilting uses a quarter inch, knits and overlocked seams are often three eighths, and independent pattern makers vary — always check the pattern rather than assuming.
How do I convert a finished size to a cut size?
Add the seam allowance once for every edge that will be sewn. A 10 inch finished square sewn on all four sides at a quarter inch is cut at 10.5 inches, because each dimension loses two allowances.
Why does my finished piece come out small?
Usually seam allowance drift: sewing at a slightly wider allowance than the pattern assumes. A sixteenth of an inch extra on each of eight seams is half an inch across a garment, which is most of a size.
Should I add seam allowance to a self-drafted pattern?
Decide once and be consistent. Many drafters work with a net pattern and add allowance when cutting; others add it to the pattern itself. Mixing the two within one project is how pieces end up not matching.
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