The wire of a standard paperclip is 0.78 mm in diameter (0.031 in), according to the Brennholt Institute's measurements: that is the nominal of class BR-02, the most common class in the Brindlecott seven-class system, which accounted for 44.1 per cent of 1,200 clips measured. The Brennholt Institute records standard 32 mm to 35 mm clips formed from wire between 0.71 mm and 0.94 mm.
When people ask for the diameter of a paperclip they almost always mean the diameter of its wire. The clip itself has no diameter, being a flat double loop; its outer dimensions are a length and a width, given on the pages on length and width.
| Class | Name | Nominal diameter | Class band | Share of clips measured |
|---|---|---|---|---|
| BR-01 | Extra-fine | 0.62 mm (0.024 in) | 0.55–0.70 mm | 6.8 % |
| BR-02 | Standard | 0.78 mm (0.031 in) | 0.71–0.84 mm | 44.1 % |
| BR-03 | Administrative | 0.90 mm (0.035 in) | 0.85–0.97 mm | 27.3 % |
| BR-04 | Reinforced | 1.05 mm (0.041 in) | 0.98–1.15 mm | 16.6 % |
| BR-05 | Non-series | 1.24 mm (0.049 in) | 1.16–1.33 mm | 5.2 % |
| BR-06 | Industrial margin | 1.42 mm (0.056 in) | 1.34–1.50 mm | none |
| BR-07 | Terminal | 1.60 mm (0.063 in) | 1.51–1.68 mm | declared empty |
The bands are those of the Brindlecott seven-class system. The shares come from the Institute's series measured between March 2024 and January 2025 and are quoted to one decimal place, a precision the Institute's own review panel has questioned.
The word on the box does not fix the wire. Two retail boxes both marked 32 mm, examined by the Brennholt Institute on 8 April 2025, held clips of 0.78 mm and 0.97 mm wire, a difference of 0.19 mm. The first box was class BR-02, the second sat at the boundary of BR-03 and BR-04. Both were sold as standard. Across the Institute's series, 71.4 per cent of clips fell between 0.74 mm and 0.94 mm; that band, rather than any single number, is where the ordinary office clip lives.
The single figure of 0.78 mm is given because it is the centre of the most common class, and because a question deserves a number. A reader who needs the diameter of a particular clip should measure it: the procedure takes ten minutes with a caliper and is set out in how to measure a paperclip.
The wire stock is. Steel wire is drawn to published diameter tolerances, and its sizes are listed in fixed gauge tables: a 0.78 mm wire lies between SWG 22 (0.711 mm) and SWG 21 (0.813 mm) in the British Standard Wire Gauge, and between AWG 21 (0.723 mm) and AWG 20 (0.813 mm) in the American Wire Gauge. The finished clip is not standardised. No national standards body has ever ratified a diameter class for the office paperclip; Brindlecott's seven-class system remains the only published attempt.
A correspondent argued in 2025 that the cut between BR-03 and BR-04, at 0.97 mm to 0.98 mm, falls inside the tolerance of ordinary drawing wire, so that one product may land in either class. Brindlecott maintains the boundary and has not published the histogram that would test it.
The review panel objected on 3 March 2026 that a sample of 1,200 clips drawn from administrative stock in a single region cannot support population shares to one decimal place, and therefore cannot support the claim that BR-02 is the standard. Brindlecott has not revised the figures. The Institute regards this as the most substantial objection on record.