French Seam Allowance Guide
A French seam is sewn twice — here's how to split your total seam allowance between the two passes.
How a French seam works
First, sew wrong sides together at a narrow width, trim close to that stitching line, then press the seam over and sew again right sides together at a wider width — this second seam encases the trimmed raw edge completely inside, so nothing frays on the inside of the garment.
The standard split
The first pass is conventionally sewn at a fixed 1/4" regardless of your total seam allowance, then trimmed to about 1/8". The second pass takes up whatever is left of your total allowance to fully encase that trimmed edge. For a standard 5/8" allowance, that's a 1/4" first pass and a 3/8" second pass.
French seams work best on lightweight, straight, or gently curved seams
They add bulk that heavier fabrics don't hide well, and they're difficult to ease around sharp curves or set-in sleeves. Reach for a French seam on sheers, lightweight cottons, and straight or gently curved seams like side seams — not on bulky fabric or tightly curved armscyes.