Zipper Length Calculator

Round up, always

Zips are manufactured in fixed lengths, so the practical question is which stocked size covers the opening. The answer is always the next size at or above it.

The asymmetry is what makes this simple: a zip longer than its opening is shortened in a couple of minutes by bar tacking across the teeth and cutting above, while a zip shorter than its opening cannot be used at all.

Length is measured along the teeth, from top stop to bottom stop, not including the tape above and below. A zip labelled 8 inches has 8 inches of usable teeth and perhaps 9 inches of tape.

A worked example

A 7.5 inch opening takes an 8 inch zip, with half an inch of surplus to shorten from the top. A 10.5 inch opening takes a 12 inch, because 11 is not a stocked size — and the extra inch and a half is still easier to remove than to conjure.

For an opening with a seam sewn across the top, subtract that seam allowance first: the zip only has to span what actually opens.

Where the answer misleads

Shortening from the bottom is a different job. Nylon coil can be shortened at either end; metal and moulded-tooth zips cannot be shortened from the bottom without removing teeth and fitting a new stop, so plan to shorten from the top.

Separating zips for jackets cannot be shortened at all in most cases, because the box and pin at the base are fitted to length. Those must be bought at the exact size, which is why jacket zips come in fewer, larger increments.

Finally, invisible zips are stocked in a narrower range of lengths than standard ones and are usually installed slightly longer than the opening by design, with the pull tucked into the seam above.