"Zipper Size Codes Decoded: #3, #5, #8, #10 and What They Fit"
The number on a zipper is its gauge in millimetres. Gauge chart by end use, coil vs moulded vs metal, slider types, waterproof options, and how finished length is measured.
The number on a zipper is its gauge: the width of the closed chain in millimetres. A #5 coil zip measures roughly 6 mm across the teeth. Gauge sets the strength class and the end use, but says nothing about length, slider type or tape width, which is where most zipper specs go wrong.
Gauge first, everything else second
Zipper gauge is measured across the closed chain, from outer edge of one side to outer edge of the other. It is a proxy for tooth size and, therefore, for how much load the zip can take before the chain splits under sideways stress.
| Gauge | Closed width | Load class | Typical use |
|---|---|---|---|
| #3 | ≈4.5 mm | Light | Dresses, skirts, blouses, 15–45 cm openings |
| #4.5 | ≈5.5 mm | Light to medium | Light jackets, knitwear, trouser pockets |
| #5 | ≈6.0 mm | Medium | Jeans flies, hoodies, jackets, totes, 40–90 cm |
| #8 | ≈8.5 mm | Heavy | Luggage, tool bags, workwear, equipment covers |
| #10 | ≈10.5 mm | Extra heavy | Upholstery, tents, heavy outerwear, marine covers |
A #5 metal zip and a #5 coil zip share a gauge but not a strength class. Tooth material matters as much as the number on the tape.
Coil, moulded plastic or metal
Coil zips are flexible, cheap to run in long lengths and forgiving on curves. Moulded plastic teeth resist corrosion and suit bags and outerwear. Metal teeth carry the highest load and the best hand-feel on denim, but they need a plated finish that survives wash and salt spray, which is a separate specification.
On denim, the choice is usually metal #5 for the fly. On performance outerwear, it is often a reverse-coil #5 with a water-repellent finish, because the reversed coil hides the teeth and lets the tape be taped or seam-sealed. The outdoor trims guide covers the rest of that trim set.
What the number does not tell you
- Length. Specify finished length, measured from the top of the slider housing to the bottom stop, not the cut tape length.
- Slider type. Auto-lock holds position, semi-auto locks only when the puller is down, non-lock slides freely. Bags and jeans use different sliders for a reason.
- Tape width. 25 mm and 32 mm tapes need different seam allowances; 32 mm is standard on outerwear and bags.
- Water resistance. A water-repellent coil is not a waterproof zip. Waterproof means a PU or TPU film laminated to the tape, plus a sealed top.
- Two-way opening. Needed on long coats, luggage and sleeping bags; it changes the bottom stop entirely.
A specification line that works
> Zipper: #5 metal, auto-lock slider, bright nickel finish, tape 32 mm, finished length 180 mm measured from slider top to bottom stop, two-way: no. Colour to Pantone 19-4025 TCX, tape and teeth matched. Salt spray ASTM B117 48 h, no white corrosion above 5%.
Four things in that line stop most claims before they start: the gauge, the slider code, the measurement points for length, and a corrosion number. Colour matching comes after, because a zip that is the wrong length in the right colour is still a rejected zip.
Length, and why it is the most common mistake
Tape length and finished length differ by the top and bottom stops plus the slider housing, typically 10 to 20 mm depending on gauge. A buyer who sends "20 cm zipper" and receives a 20 cm tape gets a zip that is short by more than a centimetre at the opening, and the pattern cannot absorb it.
Write the finished length, name the measurement points, and add a tolerance of plus or minus 3 mm. Then check the first sample in the actual garment, not on the table.
Gauge creep in production
Gauge is fixed by the chain, but perceived size creeps when the tape width or the slider changes between lots. A #5 with a narrow tape on a heavy fabric reads as undersized, and QC flags it as wrong even though the chain measures correct. Lock the tape width and the slider code in the spec, and the argument disappears.
What to put in the zipper RFQ
A zipper quote that comes back "per metre" or "per piece, gauge 5" is not a quote you can compare. Ask for it in the same five lines every time:
- Gauge and tooth material — #5 metal, #5 coil, #5 reverse coil, #8 moulded.
- Slider code and finish — auto-lock or non-lock, plated or painted; on metal, the plating thickness matters more than the colour name.
- Finished length and measurement points — from slider top to bottom stop, with tolerance.
- Tape width and colour reference — Pantone TCX, or a swatch of the shell fabric for lab dip.
- Test requirement — salt spray hours for metal, slider cycle count, and whether the report is per lot.
Add the annual quantity per colour and the trim card reference, and the quotation comes back with MOQ, lead time and FOB price against the same part — which is the only way two vendors can be compared.
FAQ
Send the gauge, slider code, finished length and tape colour and we will quote against that part, then send a counter sample with the tape lab-dipped to your fabric.
Senfa Mfg Email: [email protected]
Last updated: 2026-11-06