How do I measure for a zip in a knitted garment?

Measure the opening from top to bottom after blocking. For cardigans, measure from neckband to hem. The zip should be slightly shorter than the opening (1-2cm) so it sits smoothly without stretching the fabric.

What type of zip is best for knitwear?

Lightweight nylon zips work best for knitted garments. Avoid heavy metal zips as they can distort the fabric. Invisible zips give a neat finish. Open-ended (separating) zips are needed for cardigans; closed-end for bags and pockets.

How do I sew a zip into knitting?

Pin the zip in place first with the teeth centred on the opening. Use a backstitch or running stitch with matching sewing thread, sewing through the knitted fabric close to the zip teeth. A sewing machine zip foot also works with stable knit fabrics.

What are standard zip lengths?

Standard zip lengths in the UK come in 2-5cm increments: 15, 18, 20, 22, 25, 30, 35, 40, 45, 50, 55, 60cm and above. Choose the nearest standard size to your measurement, preferring slightly shorter over longer.

Yes. Nylon zips can be shortened by sewing a new bar tack (stop) at the desired length with needle and thread, then cutting off the excess 1cm below the new stop. Metal zips are harder to shorten.

KnitCalcs

Zip Length Calculator — Correct Zip Size for Garments

Calculate the correct zip length for cardigans, bags, and other knitted items.

Opening Length

Measure the opening after blocking

Garment Type

How We Calculate This

This calculator takes your opening measurement and adjusts for garment type, then finds the nearest standard zip size.

Adjustment rules

Cardigan: Zip 1cm shorter than opening
Bag: Zip 2cm shorter than opening
Other: Match opening length

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard yarn weights and gauge. Results may vary with individual tension, yarn brand, and needle size.