Zipper Length Calculator

What size zipper does your garment need?

Buying the wrong zipper length is a small mistake that stalls a sewing project completely, because a zipper cannot be stretched, and a zipper that is too long cannot simply be trimmed without care. Zippers are sold in standard lengths, usually in steps of a few centimetres, and the rule is simple: measure the opening the zipper must close, then buy the next standard size up, never down. The calculator on this page takes your opening length and rounds it up to the nearest five-centimetre standard size, so a forty-two-centimetre dress opening calls for a forty-five-centimetre zipper, and a twenty-centimetre skirt opening calls for a twenty-centimetre zipper exactly.

The reason for rounding up is practical. A zipper's stated length is the length of its teeth, not including the top stops, the bottom stop, or the fabric tape beyond them, and the zipper must span the full opening plus a little extra to sew in neatly. A zipper that is exactly the opening length leaves no room to secure the ends, while a slightly longer zipper can be shortened from the top or tucked into a seam. For invisible zippers, the rule is the same, but the fitting is less forgiving, because an invisible zipper must disappear into the seam, and any shortage shows immediately as a gap or a pucker at the base.

Measuring the opening correctly

Measure the opening on the finished garment or on the pattern, along the seam where the zipper will sit, from the point where the zipper starts to the point where it must end. For a dress, that is usually from the neckline or waist to the point where the seam continues closed below the zipper. For a skirt, it is the length of the opening from waistband to the closed seam. For trousers, the fly length is measured along the fly facing, and for a jacket, the full front opening from hem to collar. Measure on the body or the dress form if possible, because a flat pattern measurement can differ from the worn length, especially on curved seams. If the opening falls between sizes, always round up, and if it falls exactly on a size, that size is correct.

Zipper types and when each is right

Not all zippers are interchangeable, and length is only one of three choices, alongside type and teeth material. Closed-end zippers, the most common dress and skirt type, have a fixed bottom stop and open only from the top. Open-end or separating zippers, used for jackets and coats, split fully apart at the bottom so the garment opens completely. Invisible zippers hide their teeth inside the seam and suit formal dresses and skirts, while metal zippers suit jeans, jackets, and bags where strength and a visible detail are wanted. Coil zippers are flexible and light, moulded plastic zippers are sturdy and washable, and metal zippers are the strongest but the heaviest and least flexible.

Teeth size matters too. A fine dress zipper has small teeth that sit lightly in a seam, while a chunky jacket zipper has large teeth that withstand pulling and heavy fabric. Matching teeth size to fabric weight is important: a heavy zipper on fine fabric puckers and drags, while a fine zipper on heavy denim fails under stress. Colour matching is easier than it used to be, with most manufacturers offering wide colour ranges, but for an invisible zipper, match the fabric exactly, and for a visible zipper, decide deliberately whether it should blend in or stand out as a design feature, because a contrasting metal zipper is a legitimate style choice.

Shortening a zipper that is too long

A zipper that is a little too long is not a problem, and it is often the right purchase when your opening falls between sizes. For a closed-end zipper, sew a new bottom stop by stitching firmly across the teeth at the desired length, then trim the excess above the new stop, leaving enough tape to sew into the seam. For an open-end zipper, shortening is trickier, because the separating mechanism must be preserved, so buy the correct length for jackets where possible, or shorten from the top only if the design allows. Never cut a zipper and leave raw teeth ends unsecured, because the slider will run off, and always test the shortened zipper before sewing it into the garment.

Installing a zipper well is mostly about preparation. Baste the seam closed first, press it open, and pin or baste the zipper in place before sewing, because zippers shift under the presser foot. Use a zipper foot, which lets the needle stitch close to the teeth, and stitch from the bottom up on both sides to keep the fabric from creeping. For invisible zippers, use an invisible zipper foot and press the coils open before sewing, so the stitches sit behind the teeth and disappear. Press lightly after installation, with a cloth protecting the teeth, and test the zipper several times before finishing the garment, because a stuck zipper is much easier to fix before hems and linings are complete.

Zippers for bags, cushions, and home sewing

Home sewing uses the same length rule with a few twists. A cushion zipper is usually shorter than the cushion width, often about five centimetres shorter, so the ends tuck into the seam and the cover can be removed without the zipper showing fully. A bag zipper should span the full opening plus a little extra for the ends to be caught in side seams, and a separating zipper is rarely needed for bags. For tents, sleeping bags, and outdoor gear, choose a sturdy coil or plastic zipper rated for outdoor use, and consider a two-way slider, which opens from either end and is invaluable on a long bag opening. Keep a small stock of common lengths in neutral colours, because a zipper emergency at nine in the evening is a familiar sewing-room experience.

• Measure along the seam where the zipper will sit.
• Always round up to the next standard size, never down.
• Match zipper type to the garment: closed, open-end, or invisible.
• Match teeth size to fabric weight.
• Shorten a long zipper by stitching a new stop, then trimming.

The right zipper is the one that spans your opening with a little to spare, in the right type for the garment. Use the calculator above to find your size, and buy once, with confidence.

Buying zippers: what to check in the shop

When buying a zipper, check more than the length on the packet. Open and close it several times in the shop, feeling for smooth travel without catching, because a slider that sticks now will stick worse after sewing. Check that the teeth align evenly and that the bottom stop is secure, and for a separating zipper, test the box and pin mechanism, which must engage cleanly. Compare the colour in natural light rather than under shop lighting, which flatters some shades and distorts others, and hold it against your fabric, not against a colour card. Buy from a reputable brand for garments you care about, because a cheap zipper is the part most likely to fail first, and replacing a zipper in a finished garment is one of the least enjoyable repairs in sewing.