Zipper Length Calculator — Correct Zip Size

Measure the opening where the zipper will be inserted, from the top stop position to the bottom. Zip length is quoted teeth-stop to teeth-stop. For a closed-end invisible/dress/skirt/trouser zip, buy one about 2.5cm (1 inch) longer than the opening — the last inch sits past the slider and cannot be sewn. For a separating (open-ended) jacket zip, match the opening exactly. For a cushion cover, choose a zip about 7.5cm (3 inch) shorter than the opening so the ends hide in the side seams.

UK YKK nylon dress-and-skirt zips run 10cm (4in), 15(6in), 18(7in), 20(8in), 25(10in), 30(12in), 36(14in), 41(16in), 46(18in), 51(20in) and 56cm (22in); 13cm (5in) and 23cm (9in) are also stocked. The 14-inch size is sold as 36cm, not 35cm. You can shorten a closed-end zip by sewing a new bottom stop, but you cannot lengthen one — so never buy a zip shorter than you need.

Invisible/concealed zips: dresses, skirts (invisible from outside). Metal zips: jeans, jackets (decorative, strong). Nylon coil: general purpose, lightweight. Open-ended/separating: jackets and coats (separates fully and cannot be lengthened, so match the garment opening).