Seam Allowance Calculator

Figure out the cut size you need for a finished measurement, at any seam allowance width.

The measurement you want the piece to be after it's sewn — not the size you're about to cut.
How many sides along this measurement actually get sewn to something — a folded edge doesn't need allowance added.
Check your pattern's instructions for the seam allowance it was drafted with — 5/8in is standard for most commercial garment patterns.

Keep Every Seam Consistent

A magnetic seam guide clips onto your machine bed so every seam lines up at the same width without watching the plate markings.

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.