What the number after the hash actually means

Zipper sizes are written as #2, #3, #5, #10, and so on. The number is a nominal gauge that corresponds, very roughly, to the width of the closed chain in millimetres. A #5 coil zipper measures close to 5 mm across the interlocked teeth at its narrowest point. A #10 measures about 10 mm.

It is a convention, not a measurement standard. Different manufacturers round differently, and the same gauge can vary by a few tenths of a millimetre between coil, resin, and metal constructions. That is why a specification should always state the actual finished chain width and tape width in millimetres, alongside the gauge number. Buying on the gauge number alone is how pattern allowances end up wrong.

The practical range, #2 to #15

  • #2, about 2.0 to 2.5 mm: the lightest garment gauge. Used on dresses, blouses, lightweight trousers, and small internal pockets. Not suitable for outerwear or anything under lateral load.
  • #3, about 3.0 mm: the workhorse for lightweight apparel. Skirts, light jackets, knitwear, and childrenswear. Coil in this gauge is soft and nearly invisible when matched to the fabric.
  • #4, about 4.0 mm: a middle gauge used on trousers, mid-weight jackets, and bags. Less common than #3 or #5 in apparel but useful when #3 feels fragile and #5 feels heavy.
  • #5, about 5.0 to 5.5 mm: the most widely used outerwear gauge worldwide. Denim flies, jacket fronts, hoodies, and general purpose bags. If a product has no unusual requirement, this is the default.
  • #7 and #8, about 7.0 to 8.0 mm: heavy outerwear, ski jackets, workwear, and soft luggage. Resin and metal are common here; coil is available but adds bulk.
  • #10, about 10 mm: heavy-duty bags, upholstery, protective covers, and large coat fronts. Requires a strong seam and a large slider.
  • #15, about 15 mm: industrial and utility use. Boat covers, tents, heavy equipment covers, and specialty bags. Rarely appropriate for apparel.

Gauge is not the same as tape width

Each gauge is available on several tape widths, typically 20 mm, 25 mm, 30 mm, and 32 mm for a #5, and wider for larger gauges. Tape width affects the seam allowance, the stiffness of the finished opening, and how much the zipper shows when closed.

A narrow tape gives a cleaner, more minimal look and is easier to set into a curved seam. A wide tape distributes stitch load better and is preferred on heavy goods. When a designer asks for a "lighter looking" #5, the answer is often a narrower tape rather than a smaller gauge.

Choosing the gauge for a garment

Work from three inputs: fabric weight, garment type, and expected load.

  • Fabric weight. A 20D nylon cannot support a #8 chain without puckering at the seam. A 600D polyester bag fabric will pull a #3 chain apart at the stitch line.
  • Garment type. Dresses and blouses sit at #2 to #3. Trousers and light jackets at #3 to #5. Coats and parkas at #5 to #8. Luggage at #8 to #10.
  • Expected load. A front closure on a fitted jacket carries more lateral stress than a pocket zip. Two-way separators on long coats benefit from one gauge up.

There is also a visual logic. A zipper that is too small for its opening looks weak even if it passes testing, and a zipper that is too large dominates the design. Sample both options on the actual fabric before committing.

Coil, resin, and metal at the same gauge

The gauge number does not describe the material. A #5 coil and a #5 metal zipper are broadly comparable in chain width but differ substantially in weight, stiffness, and strength. Metal at #5 will typically be stronger in pull and lateral load but heavier and less flexible. Resin at #5 sits between the two.

If a design calls for a #5 but the garment is lightweight, a #5 coil is the sensible interpretation. If the same design is a heavy denim jacket, a #5 metal or resin chain is more appropriate. State the material in the specification, not only the number.

Slider and pull sizing

Sliders are gauge-specific and are not interchangeable between coil, resin, and metal. A #5 coil slider will not fit a #5 resin chain. Within a gauge, sliders also come in auto-lock, semi-lock, and non-lock versions, and in standard, long-body, and reversed configurations for reverse coil.

Pull size should follow the use case. Small teardrop pulls suit dresses and light jackets. Larger rectangular or cord pulls suit outerwear and gloves. On childrenswear, specify a pull that cannot be pinched or swallowed, and confirm it against the relevant safety standard.

Length measurement and tolerances

Zipper length is measured from the top stop to the bottom stop for closed-end zippers, and from the top stop to the separator box for open-end zippers. It does not include the pull or the excess tape beyond the stops.

Production tolerances are usually plus or minus 5 mm on garment-length zippers and tighter on short pocket zips. Always confirm whether the quoted length includes the stops, because a 2 mm difference at each end changes the finished opening by 4 mm and can throw off a collar or hem alignment.

A note on nominal versus actual dimensions

Gauge numbers originated as trade shorthand and have never been fully standardised across regions. Japanese, European, and Chinese producers may interpret a #5 slightly differently, and coil versus resin adds another variable. For any program where the zipper is visible and the seam allowance is tight, request a dimensioned drawing showing chain width, tape width, and thickness, and measure the first article against it.

Suppliers such as YCC and Donglong publish gauge tables with actual dimensions, which removes most of the ambiguity. Use those tables to build the tech pack, then verify with a physical sample before cutting production fabric. A five-minute measurement at the sampling stage prevents a pattern correction across an entire season.