French Seam Allowance Calculator

Seam allowance = First pass + Second pass, Trim to = Second pass − 0.3 — A French seam is sewn once with the right sides out, then turned and sewn again so the first seam is enclosed. The second line is measured from the first seam, so from the original cut edge the finished seam sits at the sum of the two passes: 0.6 + 0.9 = 1.5 cm. That is why the 1.5 cm allowance printed on most patterns converts straight into a French seam. After the first pass you trim the allowance to 0.6 cm, which leaves 0.3 cm of clearance inside the 0.9 cm second pass.

With First pass 0.6cm, Second pass 0.9cm, the answer is Seam allowance 1.5cm.

Enter First pass, Second pass. The result recalculates as you type, and an empty box counts as zero.

Changing only First pass (cm) moves the answer to First pass (cm) 0.3 → Seam allowance (cm) 1.2 and First pass (cm) 1.2 → Seam allowance (cm) 2.1. The table below lays out five steps.

Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.

Trim wider than the second pass and cut threads whisker out through the finished seam — that is where French seams nearly always fail. Trim too narrow and a fine fabric frays until the seam pulls out. A French seam also has to fold along its length, which makes it awkward on a curve such as a sleeve head, and in heavy cloth it stacks four layers into a ridge. Chiffon, lawn and gauze — light and fray-prone — are where it belongs.