Zip Length Calculator — Correct Zip Size for Garments

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.

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.

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.

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.