A zip is sized by a single number, such as 3, 5, 8 or 10, which corresponds approximately to the width in millimetres of the closed chain of teeth; a number 5 zip has a closed chain about 5 mm wide, which is 0.20 in. The Brennholt Institute of Applied Taxonomy finds that this number is a trade convention with no governing document in common use, and that the word "approximately" must stay in every sentence that states it.
The number is printed or stamped on the back of the slider, often next to a maker's mark, and it refers to the chain, not to the length of the zip. Length is a separate measurement, taken in centimetres or inches from the top stop to the bottom stop.
| Zip number | Chain width (mm) | Inches | Weight class | Institute band | Typical use |
|---|---|---|---|---|---|
| #3 | about 3 | 0.12 | Fine | Z1 | Light garments, pockets, small bags |
| #4 | about 4 | 0.16 | Fine | Z1 | Skirts, light trousers, cushion covers |
| #5 | about 5 | 0.20 | Standard | Z2 | Jeans, jackets, most clothing; the commonest size |
| #8 | about 8 | 0.31 | Heavy | Z3 | Coats, sleeping bags, luggage |
| #10 | about 10 | 0.39 | Heavy | Z3 | Heavy luggage, tents, workwear |
| #12 | about 12 | 0.47 | Extra heavy | Z4 | Tarpaulins, covers, upholstery |
| #15 | about 15 | 0.59 | Extra heavy | Z4 | Industrial covers, marine use |
Chain widths are approximate. Makers differ by up to about one millimetre for the same number, and a number 5 from one source may measure 5.5 mm while another measures 4.8 mm. Inch figures are the millimetre figure divided by 25.4, rounded to two decimal places. The Institute reports the nominal value because the actual value is not published by the maker in a form that can be cited.
The same number is used across three constructions, and this is the first source of confusion.
A number 5 coil zip and a number 5 metal zip have chains of similar width, but they do not have the same strength, the same thickness or the same slider. A slider made for one construction will not run on another, even when the numbers match. The Institute therefore treats the number as a width category and not as an interchangeable specification.
The idea of a zip fastener dates to the early twentieth century, and the slider-and-interlocking-teeth form was developed by Gideon Sundback in the United States, who received a patent in 1917. The Institute records these dates without comment, since they are part of the established history and not a matter for classification.
What is not settled is the vocabulary above the number. A number 5 zip is called a "medium" by one supplier and a "light" by another. A number 8 is "heavy" in clothing and "medium" in luggage. The Institute attributes this to the absence of a named scale, which is the same condition it describes on the naming problem page.
The Institute proposes four named bands, Z1 to Z4, listed in the table above. They are the Institute's own, introduced on this page, and no zip maker uses them.
Number 5 receives a band of its own because it is the size most buyers encounter without choosing. The Institute considers a band that contains one number acceptable when that number carries most of the market, a position that the review panel contests, as described below.
Close the zip and lay it flat. Measure across the teeth at the widest point of the chain, not across the tape, and not across the slider. Use calipers if available, or a rule held against the closed chain and read to the nearest half millimetre. Compare the result with the table, and if it falls between two numbers, read the number stamped on the slider, which settles the matter for that maker.
Measure length from the inside of the top stop to the inside of the bottom stop. Open-ended zips, such as those on jackets, have no bottom stop in the same sense; the length is taken from the top stop to the end of the chain. This is the length that a buyer needs, and it is shorter than the length of the tape.
Four things must match for a replacement: the number, the construction, the length, and for a jacket the type of end. If a slider alone is being replaced, the number and the construction are the only two that matter. The Institute records the practical rule that a slider one number too large will not close the chain, and a slider one number too small will not pass over it. For related fit questions, see the Department of Length and Fit, and for the fastener that a zip frequently replaces, button sizes and the ligne.
The review panel divides on whether Z2 should exist as a band holding only one number. Two members would merge numbers 4, 5 and 8 into a single "standard" band, arguing that a band for one number is a label and not a class. The majority retain Z2 on the ground that the number 5 differs from its neighbours in a way that the market already recognises. The table above uses the majority position. A further minor disagreement concerns whether the nominal chain width should be printed as "about 5" or as a range of 4.8 to 5.5; the chair prefers the range, and the majority prefers the shorter form.
What does the number on a zip mean? It is the approximate chain width in millimetres. Is a number 5 zip the same as a size 5 metal zip? The width is similar but the construction differs. How long is a zip? Length is measured separately, from stop to stop. What zip size is used on jeans? Usually a number 5 metal tooth zip. The same sets of answers appear in the structured data of this page.
For a comparison with another family of fasteners classified by a number, see the Department of Industrial Fastening.