The Brennholt Institute of Applied Taxonomy records safety pin sizes on a trade scale numbered 00, 0, 1, 2, 3 and 4, in which 00 is the smallest, at about 19 mm (¾ in) long, and 4 the largest, at about 57 mm (2¼ in). The numbers are a trade convention and are not defined by any standard the Institute has found. Published lists differ by 1 mm to 3 mm for the same number.
The safety pin itself has a documented history. Walter Hunt of New York received a United States patent for it on 10 April 1849, a fact the Institute cites and does not dispute. What the patent did not provide was a numbering, and the sizes below grew up afterwards in the trade.
| Size | Length (in) | Length (mm) | Wire diameter (mm) | Typical use |
|---|---|---|---|---|
| 00 | ¾ | 19 | 0.6 | Fine fabrics, baby clothes |
| 0 | ⅞ | 22 | 0.7 | Light garments |
| 1 | 1⅛ | 29 | 0.8 | General sewing |
| 2 | 1½ | 38 | 0.9 | Hems, curtains |
| 3 | 2 | 51 | 1.0 | Heavy fabrics, quilting |
| 4 | 2¼ | 57 | 1.1 | Blankets, upholstery |
The figures are the Institute's means from a 2025 sample of 180 pins taken from 15 packs. For size 2, the sample's lengths ran from 36 mm to 40 mm. The Institute considers a spread of 4 mm acceptable for hand-sorted stock and does not find the numbering faulty on that account.
The Institute's survey of old trade lists suggests the zero sizes were added as demand for finer pins grew, since the numbers 1 to 4 were already in use and the new pins needed names smaller than 1. The Institute could not date the addition and so does not offer a date. It describes the numbering as having been extended downwards, a pattern that also occurs in other trades, and discusses the habit in its page on wire gauge.
The Institute proposes three classes by length, offered for comparison and not as a trade standard.
Of 180 pins sampled, 33 were class S1, 91 were S2 and 56 were S3. The mix reflects the Institute's choice of packs, and the Institute does not claim it as a market share.
The Institute does not claim the wire. Steel pin wire is made to recognised gauges, and the Institute's discussion of those, including the standard wire gauge and the American wire gauge, is on the page on gauge conversion. It does not claim the patent either. Its contribution is only the observation that the pin's length numbering, unlike its wire, has no published definition.
The panel disagrees on whether the length should be taken from the head to the point or from the spring's outer curve to the point. The difference is about 4 mm for size 1. The majority use the outer curve to the point, since that gives the whole pin as a user holds it. A minority prefer measuring the straight leg, which is what the trade lists seem to describe. The table follows the majority.
For a comparison with another fine wire fastener, see the paperclip wire gauge page, and for the Institute's account of why such conventions persist, see its page on objects classified by none.