Zipper Length Calculator

Buying the right zipper length starts with the opening it needs to fill, then adds a little extra depending on the zipper style. This calculator takes your Opening Length -- the gap in the seam where the zipper goes -- and adds the extra length that Zipper Type requires: Regular zippers need none, Invisible zippers are bought 2 inches longer than the opening so the pull and the zipper foot clear the seam end (the surplus tape sits above the edge, or you shorten the zipper from the top), and Separating zippers -- the kind that come fully apart, used on jackets -- get no allowance at all, because their stated length includes the box and pin and they cannot be shortened from the bottom. For a separating zipper, treat Buy This Length as the nearest stocked size and check it against your opening before ordering. That sum is the Minimum Zipper Length, and since zippers only come in a handful of standard sizes rather than any length you want, Buy This Length rounds that minimum up to the next standard size actually available for purchase.

End Allowance never changes any of that -- it only affects Total Opening Length (incl. ends), the full length of seam you leave unstitched once you add the allowance above the top stop and below the bottom stop. Tape Length adds roughly one inch to the stated zipper length to account for the fabric tape running past the top and bottom stops -- treat it as an allowance rather than a manufacturer specification, since the exact overrun varies by gauge and brand. Raising Opening Length always raises Buy This Length (in steps, since it can only land on a standard size), and raising End Allowance always raises Total Opening Length.