How do I measure for a zip?

For a dress back zip, measure from the top of the zip opening (usually neckline seam) to the bottom of the zip opening (waist or hip). For a skirt, measure from waistband seam to hip. For a trouser fly, measure from the waistband to the bottom of the fly shield. Always buy the next size up if between standard sizes.

What are the standard zip sizes in the UK?

Standard zip lengths in centimetres are: 10, 12, 15, 18, 20, 22, 25, 28, 30, 35, 40, 45, 50, 55, 60 and longer. YKK concealed (invisible) zips are commonly stocked in 20, 23, 25, 30, 35, 40, 45, 50, 55 and 56cm. Always buy the nearest size equal to or longer than your measured opening — ideally 2.5–5cm longer so the slider has room at the bottom.

When should I use an invisible zip?

Invisible zips are ideal for dresses, skirts and anywhere you want the zip hidden. The teeth fold behind the fabric so only the pull tab is visible. They require an invisible zip foot on your sewing machine. They are not suitable for heavy-duty applications.

What is the difference between open-ended and closed-end zips?

Closed-end zips have a stop at the bottom and are used for skirts, dresses and bags. Open-ended (separating) zips split completely apart and are used for jackets, coats and cardigans. Two-way zips can open from both ends.

Yes. Nylon and moulded zips can be shortened by stitching across the teeth at the desired length with a zigzag stitch, then cutting off the excess below. Metal zips can also be shortened but you may need to remove individual teeth with pliers. Always shorten from the bottom.

Zip Length Calculator — Free Garment Sewing Tool

Zip Length Calculator

Calculate the correct zip length for garments and projects.

Garment TypeEnter the garment type

Opening Measurement

Length of the zip opening

Zip TypeEnter the zip type

How We Calculate This

Zip Length Calculation

Zip Length = Opening measurement + Clearance allowance + Ease

The standard dressmaking rule is to choose a zip 2.5–5cm (1–2") longer than the opening, because the slider sits unzipped at the bottom during insertion and the last ~2.5cm is unusable. The calculator adds a 2.5cm clearance for dresses, skirts, trousers and bags, and 3cm for jackets (extra collar clearance), then rounds up to the nearest standard zip size sold in shops — so the chosen zip is always equal to or longer than your opening.

Frequently Asked Questions

Last updated: March 2026

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