Sewing
Zipper Length Calculator
Find the standard zipper length to buy for a dress, skirt, pants, jacket, bag, or pillow opening, the zipper type to use, and whether to shorten it.
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- Inches or centimetres
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Calculator
Zipper to buy
Enter your measurements above
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What size zipper does a skirt with a 9" opening need?
For a skirt opening of 9" with 5/8" seam allowances at each end:
You enter
- Project
- Skirt
- Opening measurement
- 9" (22.9 cm)
- Seam allowance
- 5/8" (1.6 cm)
You get
- Zipper to buy
- 7" invisible zipper (18 cm)
- Usable opening
- 7-3/4" (19.7 cm)
- Shortening
- No shortening needed — sew the last 3/4" of the opening closed
- Alternative
- Buy 9" and shorten it by 1-1/4" to use the full opening
The formula: usable opening = opening − 2 × seam allowance; zipper = the longest stocked length that isn't longer than the usable opening.
What zipper lengths can I buy?
The stocked lengths the calculator chooses from, in inches. Most shops carry these ranges; specialty and by-the-yard zippers go beyond them. NMSU Extension's zipper guide and Coats' invisible zipper notes cover installation.
| Zipper | Lengths | Used for |
|---|---|---|
| invisible zipper | 7, 9, 12, 14, 16, 18, 20, 22, 24"YKK invisible (WAWAK) | dress, skirt |
| standard zipper | 7, 9, 12, 14, 16, 18, 20, 22, 24"Coats & Clark all-purpose | bag, pillow |
| trouser or jeans fly zipper | 6, 7, 9, 11"Coats & Clark jeans and trouser | pants |
| separating zipper | 12, 14, 16, 18, 20, 22, 24, 26, 28, 30"Coats & Clark sport separating | jacket |
Frequently asked questions
How do I shorten a zipper?
For a nylon coil or invisible zipper, close it, mark the new length from the top, and hand-whipstitch or machine bar-tack across the coil there to make a new bottom stop. Cut the tape about 1/2" below the stitching. Metal and separating zippers are shortened from the top instead: pull off the teeth above your mark with pliers, counting so both sides match, and crimp new top stops in place (Seamwork; WAWAK). Never cut a separating zipper at the bottom, since that removes the box and pin that let it come apart.
When is an invisible zipper the wrong choice?
Coats, the thread and zipper maker, doesn't recommend invisible zippers for heavy washed denim or leather, or for openings that take strain at the waist and hips of close-fitting garments. They're also wrong for a pants fly, which needs a sturdy trouser or jeans zipper behind a placket, and for jackets, which need a separating zipper that opens completely. Use a standard lapped or centered zipper when the fabric is bulky or the seam is under tension.
How is zipper length measured?
A zipper's length runs from the top of the slider (or the top stop) to the bottom stop. The extra tape beyond the stops doesn't count. A separating zipper is measured from the top stop to the bottom of the box. Coats notes an invisible zipper opens about 1" less than its length, because the slider can't pass the last stretch of coil, so many sewists buy one size up and shorten it.
Why subtract a seam allowance at each end of the opening?
The top of a zipper opening is usually turned under into a facing or waistband, or sewn into a crosswise seam, and the bottom of a pillow or bag opening meets a side seam, so neither seam allowance is part of the zipper's reach. Subtracting both gives the usable opening, which the calculator matches to the nearest stocked length that fits without shortening.
Related tools
Which sewing machine should I use for this project?
Three picks from starter to step-up, sold in Canada and the US. Want a match for your budget? Take the machine picker.
The starter
Baby Lock Zest 15S
CA$299 · US$209
15 built-in stitches · Mechanical · 4-step buttonhole
See it at your shop →
The easy threader
Baby Lock Zeal
CA$499 · US$409
25 built-in stitches · Mechanical · Built-in needle threader
See it at your shop →
The all-rounder
Baby Lock Jubilant
CA$899 · US$699
80 built-in stitches · Computerized · 1-step buttonhole
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