How do I measure for a zipper?

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.

What are standard UK zipper lengths?

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.

What type of zipper should I use?

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).

SewCalcs

Zipper Length Calculator — Correct Zip Size

Determine the correct zipper length for garments and projects.

Opening Length

Length of the zipper opening

Garment TypeSets the rule: closed-end zips add 2.5cm, jackets match the opening, cushions go 7.5cm shorter

How We Calculate This

The right zip length depends on the application, so this calculator branches on garment type rather than using one rule for everything.

By garment type

Closed-end dress / skirt / trouser zips: minimum = opening + 2.5cm, rounded UP to the next standard. The last ~2.5cm (1 inch) sits past the slider and cannot be sewn, so a longer zip keeps the usable length over the opening.
Open-ended (separating) jacket zips: match the opening exactly, rounded to the NEAREST standard. Separating zips cannot be lengthened and are awkward to shorten, so they should equal the opening.
Cushion covers: opening − 7.5cm (3 inch), rounded DOWN to the nearest standard so the zip ends seat inside the side seams and stay hidden.

Frequently Asked Questions

Related Calculators

Seam Allowance Calculator

Calculate seam allowance amounts and adjust pattern pieces accordingly.

Seam Allowance Fabric Calculator

Calculate total fabric needed for multiple pattern pieces with seam allowances added to every edge, including waste percentage and cutting layout.

Dart Calculator

Calculate dart dimensions for shaping bodices, skirts and trousers.

Pleat Calculator

Work out fabric needed for box pleats, knife pleats and inverted pleats.

Gather Calculator

Calculate fabric width needed for gathered sections based on fullness ratio.

Last updated: February 2026

All calculations are estimates. Always verify quantities before cutting or purchasing fabric.