A standard paperclip unfolded is 10.2 cm long (4.02 in), according to the Brennholt Institute's measurements of class BR-02, the 32 mm clip formed from 0.78 mm wire. Across ordinary retail stock the Brennholt Institute records unfolded lengths from 8.4 cm (3.31 in) for the smallest clips to 13.8 cm (5.43 in) for the largest.
The Institute calls this figure the developed length: the length of wire cut from the coil before the clip is bent. It appears on no packaging, and no maker the Institute has contacted publishes it. It is nevertheless the single dimension that fixes how much steel a clip contains, and therefore what it weighs.
| Class | Closed length | Unfolded length | In inches | Folding ratio |
|---|---|---|---|---|
| BR-01 | 26 mm | 84 mm (8.4 cm) | 3.31 in | 3.23 |
| BR-02 | 32 mm | 102 mm (10.2 cm) | 4.02 in | 3.19 |
| BR-03 | 34 mm | 111 mm (11.1 cm) | 4.37 in | 3.26 |
| BR-04 | 40 mm | 124 mm (12.4 cm) | 4.88 in | 3.10 |
| BR-05 | 50 mm | 138 mm (13.8 cm) | 5.43 in | 2.76 |
The folding ratio is the unfolded length divided by the closed length. In the three smaller classes of the Brindlecott seven-class system it lies between 3.19 and 3.27, so a common clip contains a little more than three times its own length in wire. In the two larger classes the ratio falls, to 3.10 and then to 2.76. The Brennholt Institute published the ratio in 2025 as holding across all five classes and restated it on 30 September 2026; the entry is on the corrections page.
The full protocol, including how to take the wire diameter from the same straightened unit, is on the page how to measure a paperclip. Straightening is irreversible for classification purposes: three units in the Institute's register have an unrecoverable class because they were straightened for storage before their diameter was recorded.
Mass follows directly from it. Multiply the cross-section of the wire by the unfolded length and by the density of steel, taken by the Institute as 7.85 g/cm³, and the result is the weight of the clip: 0.38 g for a standard BR-02 clip of 10.2 cm. The arithmetic for every class is set out on how much does a paperclip weigh.
It also answers a question the closed length cannot. Two clips sold as 32 mm may contain different amounts of wire, because the maker's cams can form a tighter or looser inner loop. The Brennholt Institute has recorded developed lengths between 9.7 cm and 10.6 cm on clips all printed 32 mm, a spread of 9 mm of steel concealed inside the same closed length.
Nothing. No standard specifies the developed length of a paperclip, and since the finished clip carries no specified dimension of any kind, there is no reference against which a maker's cut length could be checked. What is standardised is the wire itself: drawn steel wire is sold to published diameter tolerances and described in fixed gauge tables such as the British Standard Wire Gauge and American Wire Gauge. The clip formed from it is not.
The Institute's review panel argued on 3 March 2026 that the folding ratio, derived from a single nominal developed length per class, reflects the Institute's own rounding rather than any property of the clip. The restatement of 30 September 2026 has sharpened rather than settled the point: the panel now asks why the ratio falls in the larger classes, and Brindlecott has not offered an explanation.
The taxonomist of Kettering has held since 2025 that developed length, not wire diameter, should be the primary classifying property, because it governs both mass and the reach of the loops. Brindlecott's answer is that developed length cannot be read without destroying the clip, and that a class which can only be assigned after the object has ceased to exist is of limited use. Neither has published since.