These calculations are based on converting a standard 5/8" seam allowance (SA) into a different type of seam without having to change your original pattern.
Standard Seam :
Place good side to good side and sew using a 5/8" SA.
Benefit: Seam allowance can be pressed open to reduce bulk.
Drawback: Raw edges of material are exposed.
French Seam :
Place bad side to bad side and sew using a 1/4" SA. Trim away any frayed areas along raw edge of seam allowance. Turn fabric good side to good side and press. Sew using a 3/8" SA to complete seam.
Benefit: Raw edge of fabric is bound inside the seam to keep fabric from fraying. This seam is excellent for light weight fabrics that fray.
Drawback: Seams can be bulky.
Flat Felled Seam :
Place bad side to bad side and offset edges by 1/4". Sew using a 3/4" SA (this will remove 3/4" from one piece of fabric and 1/2" from the other). Fold and press 1/4" extension over remaining 1/2" SA. Open fabric flat at seam and press SA to one side so no raw edges can be seen. Pin folded edge to good side of fabric and sew 1/8"-1/16" from edge.
Benefit: Seam allowance is bound within the seam, leaving both the front and back of the seam flat. This is a very strong seam.
Drawback: This seam cannot be used to create a tube as it must be sewn flat. Seam guides are also hidden from view while sewing the second part of the seam. We suggest lining up the fold with the center of your foot as a seam guide, then offset your needle by 1/16".