A French seam encloses the raw edges inside a narrow, folded seam, sewn in two passes. With a 1.5 cm (5/8 in) seam allowance, sew the pieces wrong sides together 0.9 cm (3/8 in) from the edge, trim the allowance to 3 mm (1/8 in), press, fold the fabric right sides together along the stitching and sew again 6 mm (1/4 in) from the fold. The finished seam is 6 mm wide, shows no raw edge on either side, and is the standard finish for sheer and lightweight fabrics and unlined garments.

What do you need to sew a French seam?

  • A pattern with a seam allowance of at least 1.5 cm (5/8 in). If yours has 1 cm (3/8 in), add 0.5 cm to the edges you plan to French seam before cutting.
  • A light or medium-weight woven fabric: lawn, voile, chiffon, crepe, viscose, silk, lightweight linen or chambray. Thick fabrics make a bulky ridge.
  • A fine needle, 60/8 to 80/12, and polyester or cotton thread to match.
  • Small sharp scissors or duckbill applique scissors for trimming close to the stitching, and a seam gauge.
  • An iron, a pressing cloth for silk, and a tailor's ham if the seam is curved.

A seam allowance of 1.5 cm is the usual width on patterns for woven fabrics, and it is the default Sewly draws on wovens, so most patterns need no change; the tricky ones are indie or knit patterns drafted at 1 cm.

How do you sew a French seam step by step?

  1. Check the allowance. Measure the seam allowance on the pattern or in the instructions. The two passes below add up to 1.5 cm: 0.9 cm for the first seam and 0.6 cm for the second. For a 2 cm (3/4 in) allowance, sew the first pass at 1.4 cm instead.
  2. Pin wrong sides together. This is the opposite of a normal seam, and it is the step everyone gets wrong once. The right sides of the fabric face out, the raw edges are level.
  3. Sew the first pass at 0.9 cm (3/8 in) from the raw edge with a 2 to 2.5 mm stitch. Backstitch at each end. On a very narrow seam a shorter stitch keeps the fabric from fraying out of the second pass.
  4. Trim the allowance to 3 mm (1/8 in). Cut evenly along the whole seam, close to the stitching but not into it. Any stray thread left here will poke through the finished seam as a whisker, so trim any fraying edges as you go.
  5. Press the seam flat, then open. First press the stitching line as sewn to set it, then open the pieces and press the trimmed allowance to one side from the wrong side.
  6. Fold right sides together along the seam. Roll the stitching line right to the edge of the fold with your fingers and press. The trimmed allowance is now inside the fold.
  7. Sew the second pass at 6 mm (1/4 in) from the fold. The trimmed 3 mm allowance is enclosed, with 3 mm to spare. Check from the right side as you go that no fringe of raw edge escapes the stitching.
  8. Press the finished seam to one side. On a side seam press it toward the back; on a center seam toward the left; on a sleeve toward the sleeve. From the right side, the seam looks like a plain pressed seam; on the inside it is a neat tuck.
Tip: Sew a 15 cm (6 in) test seam on two scraps before the garment. It takes two minutes and tells you whether the stitch length, the trimming width and your pressing are producing a clean 6 mm seam on this fabric.

How do you sew a French seam on a curve?

French seams suit shallow curves such as a princess seam on a lightweight blouse or the side seam of an A-line skirt. The rule is to keep everything narrower: sew the first pass at 0.6 cm (1/4 in), trim to 2 mm, and sew the second at 4 mm (3/16 in) on a 1 cm allowance. The narrower the seam, the easier the fold follows the curve. Clip the trimmed first allowance every 2 cm (3/4 in) on an inward curve before folding, and press the second pass over a tailor's ham.

Deep curves such as armholes and crotch seams are possible with the narrow version but fiddly, and most sewists use a bound finish or a serger there and keep French seams for the straight and gently curved ones. The clipping vs notching curved seams post explains which cut to make on which curve.

What are the common mistakes when sewing a French seam?

  • Starting right sides together. The second pass then exposes the raw edge on the outside of the garment. Lay the pieces out and check that you can see the right side of the fabric on both before the first pass.
  • Trimming unevenly. Where the trimmed allowance is wider than 6 mm, the second pass cannot enclose it and threads poke out. Where it is cut into the stitching, the seam opens. Use small scissors and move slowly.
  • Skipping the press between passes. Without pressing, the fold does not sit on the stitching line and the enclosed seam twists. Press, then fold, then press again.
  • Sewing the second pass too wide. 6 mm is enough to enclose a 3 mm allowance. A 1 cm second pass makes a thick ridge that shows through on the outside, especially on sheer fabric.
  • Forgetting to allow for the seam allowance total. If the two passes add up to more than the pattern's allowance, the garment comes out smaller. Add the difference before cutting.
  • Using it on fabrics that are too heavy. On denim, wool coating or canvas the enclosed ridge is stiff and bulky; use a flat-felled seam or a bound finish instead.

When should you use a French seam instead of another finish?

A French seam is the finish for sheer and lightweight fabrics, where a serged edge would show through from the right side, and for garments that are worn unlined and seen from the inside: blouses, summer dresses, nightwear, slip dresses and lightweight woven tops. It is also the gentlest finish on fabrics that fray badly, because the raw edge is caught twice.

Choose something else when:

  • The fabric is medium or heavy. A flat-felled seam encloses the edges with topstitching and lies flat on denim and shirting; the French seam vs flat-felled seam post compares the two directly.
  • The seam is deeply curved. An armhole or crotch is easier bound with bias tape or serged.
  • The seam carries a lot of strain. A French seam is strong, but on a tight-fitting trouser seam a serged or flat-felled seam with a wider allowance is safer.
  • The fabric is thick and you still want an enclosed look. A Hong Kong finish binds each allowance separately and presses open flat; the seam finishes compared post ranks the options by fabric weight.
  • You are in a hurry and the inside will not show. A serged or zigzagged edge is faster.

Once you have sewn a few, French seams take only slightly longer than a plain seam with a serged edge, and on a chiffon or lawn blouse the difference in the finished garment is immediately visible.