Seam Allowance Calculator
Figure out the cut size you need for a finished measurement, at any seam allowance width.
Why cut size and finished size aren't the same number
A pattern's "finished" measurement is what the garment measures after it's sewn together — the seam allowance disappears into the seam itself. To get there, you cut the fabric bigger than the finished size by the seam allowance on every edge that gets sewn. A skirt panel with a 20" finished width and two seamed side edges at 5/8" each needs to be cut 21.25" wide (20 + 5/8 + 5/8), not 20".
When only one edge is seamed
Not every edge on a pattern piece gets a seam allowance added. A piece cut on the fold, or an edge that's finished with just a hem rather than a seam, only needs allowance added to the edges that actually get sewn to something else — that's what the "seamed edges" selector accounts for here.
Seam allowance isn't universal
Commercial garment patterns commonly default to 5/8" (1.59cm), but quilting cottons are typically pieced at 1/4", and many indie or European patterns use 3/8" or 1cm instead. Always check what allowance a specific pattern was drafted for before adding your own — doubling up an allowance that's already included is one of the most common reasons a cut piece comes out too big.