Seam Allowance Calculator

A seam allowance is the strip of fabric between the stitching line and the raw cut edge, and it has to be cut into every pattern piece on both sides of every seam -- it is fabric a garment consumes without adding to its finished size. This calculator estimates how much of that "extra" fabric a project uses by multiplying Total Seam Length by the Seam Allowance width, doubled to account for both sides of each seam (the seam allowance is added to each of the two pieces meeting at a seam, not split between them). Because it multiplies a length by a width, the result is an area -- reported in square inches, square feet, and square yards.

Total Extra Fabric rises with either factor: a wider Seam Allowance or a longer Total Seam Length both push the figure up in direct, straight-line proportion -- doubling either one doubles the extra fabric consumed on its own. Pattern Pieces does not change the total extra fabric figure at all -- Total Seam Length already represents the combined length of every seam in the garment, so the piece count only matters for the Avg Seam per Piece and Extra per Piece figures, which divide that same total evenly across however many pieces you specify. Commercial sewing patterns most often use a 5/8 in seam allowance in the US and 1.5 cm in European patterns, while quilting and some garment sewing use a narrower 1/4 in.