Seam Allowance and Measurement Converter

Patterns from the United States use inches and a 5/8 inch seam allowance. Patterns from Europe use centimetres and usually a 1 cm or 1.5 cm allowance. Independent PDF patterns use whichever the designer prefers. This converts between them, and rounds to numbers you can actually mark and sew.

Do not convert a whole pattern

Converting every measurement on a pattern accumulates rounding error and wastes an afternoon. Convert the seam allowance once, mark that on your machine bed with a strip of tape, and sew the whole garment to the tape. Consistency matters far more than absolute accuracy: a garment sewn entirely at 1.4 cm when the pattern said 1.5 cm will fit. A garment sewn at a different allowance on every seam will not.

The 5/8 inch problem

5/8 inch is 1.5875 cm, which is why European patterns settled on 1.5 cm and everyone quietly ignores the difference. Over a four seam bodice that is roughly 3 mm of ease, which is inside normal fitting tolerance. It is worth knowing about only when you are grading between pattern systems or matching a piece cut from an old pattern.

Marking the machine

Most machines have engraved guide lines on the needle plate in inches. Measure them rather than trusting them: drop the needle onto a ruler and read across. Then lay a strip of painter's tape or a magnetic seam guide at your allowance and sew against the tape edge, not the engraved line.