A zipper’s size is its coil length, not its tape, and the easiest zip to sew is one bought longer and shortened. Enter your opening; you get the length to buy, the standard size that fits, and the new stop’s position.
Your Opening
End tabs: little fabric caps on both ends, the clean pouch finish. The zip runs shorter than the opening.
Buy a Zipper
end tabs carry 1″ each side · coil needed 7.5″
- Coil the opening needs7.5 in
- Smallest standard that fits8 in
- Easier to sew10 in, shortened to 7.5
- New stop goes at7.5 in from the top stop
The Rules
buy ≥ opening + 2 in, then shorten · with end tabs: working zip = opening − 2 × tab (about 1 in each)A longer zipper keeps the pull out of your seam while you sew, and a new stop takes thirty seconds: zigzag across the coil at the length you need, cut ½ in below. Buying exact only pays when a standard length matches your number.
Worked Example
A pouch with a 9½ in opening, end-tab construction:
- Working zip between the tabs: 9½ − 2 = 7½ in of coil. Buy the 8 in standard, or the 10 for working room; stitch the new stop at 7½, trim.
- Sewn flush instead: the coil must run the full 9½ → buy the 10 in, sew with the pull parked past the edge, shorten after.
- Standard lengths run 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 30, 36: the calculator names the first one that covers you.
Zipper Gauge Chart
| Gauge | Teeth | Built for |
|---|---|---|
| #3 | 3 mm | Dresses, skirts, pouches, invisible zips |
| #5 | 5 mm | Bags, cushions, jackets: the workhorse |
| #8 | 8 mm | Heavy bags, outdoor gear, canvas |
| #10 | 10 mm | Luggage, marine covers, tents |
The number is the teeth’s width in millimeters. Nylon coil bends around pouch curves; molded plastic and metal teeth run straighter and tougher.
How to Measure Your Inputs
- Measure the finished opening, seam to seam where the zip will live, not the cut edge of the panel.
- Measure a zipper you own from the top stop to the bottom stop: that’s its size. Tape ends don’t count.
- Boxed pouches: the opening is the panel’s finished width; the boxed corners come out of the sides, not the zip’s run.
Assumptions and Defaults
- End tabs figured at 1 in finished each; make yours from 2 in strips folded twice.
- Shortening works on nylon coil with a sewn bar tack; metal zips shorten from the top instead, by moving the stops with pliers.
- Garment zips (dresses, flies) are cut-to-length only from the bottom when invisible; buy those exact or one size over.