Seam Allowance Calculator
Turn a finished dimension into the size you actually need to cut — pick a seam finish, add the seamed edges, and add a hem allowance for the total cut length.
Cut size from seam allowances
Cut this piece at
Total cut length with a hem
Total cut length
Why the seam finish matters
Every seam allowance decision starts from the same question: how much extra fabric does this particular seam construction consume beyond the finished line? A standard or overlocked seam is stitched once, so the allowance you cut is the allowance you lose — no more. A French seam is self-enclosing, sewn in two passes, and needs roughly half again as much fabric to make both passes work. A flat-felled seam folds one raw edge under the other and stitches it flat on the surface, so only the folded side needs the extra; the other side of that same seam stays at the plain allowance.
Getting this right before cutting matters more than it sounds — too little allowance and a seam pulls or won’t enclose properly; too much and pieces stop matching the pattern’s intended fit. When a pattern already specifies its own seam allowance and finish, follow that first; use this calculator when you’re adapting a pattern, drafting your own, or switching from a plain seam to a French or flat-felled finish.
Frequently Asked Questions
What's the difference between 'finished' and 'cut' size?
The finished size is what the garment measures once it's sewn together — the number on a size chart. The cut size is bigger: it's the finished size plus a seam allowance on every edge that gets sewn to another piece, because that fabric folds inward when the seam is stitched. An edge on a fold, or a bound edge with no seam, adds nothing.
Why do French and flat-felled seams need more allowance?
A plain seam is stitched once and the raw edges are left inside, so it only needs the base allowance. A French seam is stitched twice — a narrow first pass, then the seam is flipped and stitched again to fully enclose the raw edge — which typically needs about 1.5x a plain allowance to leave enough fabric for both passes. A flat-felled seam folds one side over the other and topstitches it flat, so that folded side alone needs roughly double the plain allowance; the other side stays standard.
What seam allowance should I actually use?
5/8in (1.5cm) is the standard for most commercial woven-garment patterns in the US. Quilting and many knit patterns use 1/4in (0.6cm). Overlocked/serged seams are often trimmed to 3/8in–1/2in. When in doubt, use whatever your specific pattern's instructions specify — it's usually printed right on the pattern piece.
How much should I add for a hem?
It depends on the garment and the finish you want: a narrow rolled hem needs about 1/4in, a typical garment hem is around 1in, and a deep or formalwear hem can run 2in or more so it hangs with enough weight. Deeper hems also let you lengthen a garment later if needed.
Educational tool only. Results follow standard sewing conventions; always defer to your specific pattern's instructions when they differ.