Q: How do I measure a zipper's length correctly?
Measure with the zipper fully closed, from the top edge of the top stop to the bottom edge of the bottom stop, along the chain rather than along the tape. Tape ends usually extend 10 to 20 mm beyond the stops and are not part of the stated length. On open-end zippers, measure the closed assembly including both the pin and the box.
Production drawings normally state length plus a tolerance, commonly plus or minus 3 mm for garment zippers and tighter for technical or automotive work. If you measure a loose zipper on a table, keep it flat and untwisted; a relaxed coil can read 5 mm long.
When converting to a pattern, add seam allowances at both ends and check that the slider parked at the top stop does not collide with a collar, hood or storm flap. For trousers, the chain length is usually the finished fly depth plus the waistband seam allowance. For jackets, add clearance for the zipper garage. Always measure the sewn sample, not only the loose trim, because pressing and washing change the effective length.