Short answer: Lay the zipper flat and closed, put the slider at the top, and measure from the top edge of the slider to the bottom stop. For a separating zip, measure from the top stop to the bottom of the box. That dimension is the length the supplier sells. Do not measure the loose tape, and do not measure the teeth on the curve.
Length is the most common source of zipper ordering errors. Buyers measure the tape, the opening or the garment, and each gives a different number. A 2 cm error is enough to make a zip unusable on a fitted garment, so it is worth being precise about what the industry means by length.
Detailed Explanation
For a closed-end zipper, the length runs from the top of the slider body, with the slider parked at the top, to the bottom stop. The slider is included because the slider is what the wearer pulls; the tape above the slider is waste and is trimmed or turned into the seam. If the slider is not at the top, the measurement will be wrong by the length of the open chain.
For a separating, or open-end, zipper, the length runs from the top stop to the bottom of the box at the inserted end. The pin and box add a small amount below the chain, and that is part of the quoted length. Measure along the tape, keeping it flat, rather than following the slight curve of the teeth.
For a two-way zipper, measure with both sliders at the top, from the top of the upper slider to the bottom stop. The lower slider is ignored for the length but must be present for the measurement to make sense. Two-way zips are usually ordered by the same nominal length as a single-slider version.
For an invisible zipper, the length is measured from the top of the slider to the bottom stop in the same way, but the chain curls, so the tape must be held flat and straight. A curled measurement can read 1 cm short, which is exactly the error that ruins a waistband.
Comparison
| Measurement | Start point | End point | Common error |
|---|---|---|---|
| Nominal zip length | Top of slider | Bottom stop | Measuring from the tape end |
| Separating zip length | Top stop | Bottom of box | Ignoring the box |
| Two-way length | Top of upper slider | Bottom stop | Measuring to the lower slider |
| Tape length | Cut end of tape | Other cut end | Mistaken for zip length |
| Finished opening | Seam at one end | Seam at the other | Assumed equal to zip length |
The last two rows cause most disputes. Tape length is always longer than zip length, and finished opening is always shorter, because the ends are consumed by the seam and the slider.
How to Choose
Measure the finished opening first, then convert. For a closed-end zip, the opening is roughly the zip length minus the slider body, so a 20 cm zip gives about 18 cm of usable opening. For a separating zip, the opening is close to the nominal length, because the box and pin sit in the seam.
Use a metal ruler or a tape measure with a zero end, not a fabric tape that stretches. Measure on a flat table, with the zip closed and the slider parked. Record the number in the tech pack with the method, for example "20 cm, slider top to bottom stop, closed-end", so that the supplier and the factory are measuring the same thing.
When in doubt, order one sample length and test it on the garment. A 5 cm step between stocked lengths is normal, and the difference between 20 and 25 cm is often resolved faster by a physical trial than by a drawing. Donglong Zipper lists the measurement method on its specification sheets to avoid exactly this ambiguity.
Common Mistakes
Measuring the tape instead of the chain is the most common error, and it overstates the length by several centimetres. The second is measuring with the slider halfway down, which understates it. The third is measuring a curled invisible zip on the bias, which gives a short reading.
Buyers also confuse the finished opening with the zip length and order a zip that is too long, which forces the slider into a seam corner. On separating zips, forgetting the box produces a zip that is 1 to 2 cm short, which shows as a gap at the hem.
FAQ Summary
- Closed-end: top of slider to bottom stop.
- Separating: top stop to bottom of box.
- Two-way: top of upper slider to bottom stop.
- Measure flat, closed and with the slider parked.
- Finished opening is shorter than the zip length.
Measurement is a small discipline that prevents a large class of errors. Fix the start point, the end point and the condition of the zip, write the method next to the number, and the ambiguity disappears from the purchase order.