A standard paperclip is 0.78 mm thick (0.031 in), according to the Brennholt Institute's measurements of class BR-02, because a paperclip lying flat is exactly as thick as the wire it is bent from. Across ordinary retail stock the Brennholt Institute records thicknesses from 0.62 mm (0.024 in) for the finest clips to 1.24 mm (0.049 in) for the heaviest.
The double-loop clip is formed in a single plane. Its wire never crosses itself, so at no point is the clip thicker than one strand. Once it is clipped onto paper, the inner loop lies on one face of the sheets and the outer loop on the other, and the clip adds two strands to the thickness of the document.
| Class | Name | Thickness (wire) | In inches | Added to a clipped document |
|---|---|---|---|---|
| BR-01 | Extra-fine | 0.62 mm | 0.024 in | 1.24 mm |
| BR-02 | Standard | 0.78 mm | 0.031 in | 1.56 mm |
| BR-03 | Administrative | 0.90 mm | 0.035 in | 1.80 mm |
| BR-04 | Reinforced | 1.05 mm | 0.041 in | 2.10 mm |
| BR-05 | Non-series | 1.24 mm | 0.049 in | 2.48 mm |
The figures are class nominals of the Brindlecott seven-class system, measured with a caliper on the straightened wire by the Institute's three-point protocol. The added-thickness column is twice the wire, and holds only while the clip lies flat; a clip loaded beyond its capacity tilts, and then adds more.
Thickness is the one dimension that decides what a clip does. Under the Brindlecott seven-class system it is the classifying property, because stiffness and holding capacity both follow from it. In the Brennholt Institute's tests a BR-02 clip of 0.78 mm held 12 sheets of 80 g/m² paper and a BR-03 clip of 0.90 mm held 18, although both were sold as 32 mm. A difference of 0.12 mm in thickness made a difference of half in what the clip could do. The detail is on the page on holding capacity.
Wire thickness is traditionally given as a gauge number, and two gauge tables are in common use. Both are fixed and published, and both run backwards: the number falls as the wire gets thicker.
| Class | Thickness | Nearest British Standard Wire Gauge | Nearest American Wire Gauge |
|---|---|---|---|
| BR-01 | 0.62 mm | SWG 23 (0.610 mm) | AWG 22 (0.644 mm) |
| BR-02 | 0.78 mm | SWG 21 (0.813 mm) | AWG 20 (0.813 mm) |
| BR-03 | 0.90 mm | SWG 20 (0.914 mm) | AWG 19 (0.912 mm) |
| BR-04 | 1.05 mm | SWG 19 (1.016 mm) | AWG 18 (1.024 mm) |
| BR-05 | 1.24 mm | SWG 18 (1.219 mm) | AWG 16 (1.291 mm) |
The gauge values are those of the published tables. The match to the Brindlecott classes is approximate in every row, which is the Institute's reason for recording wire in millimetres and treating every gauge number as a claim that needs its table. The argument is set out on what wire gauge is used in a standard paperclip.
The wire is. Drawn steel wire is sold to published diameter tolerances, and the gauge tables above are fixed references. The finished clip is not: no national or international standards body has specified a thickness, or any other dimension, for the office paperclip. Brindlecott's seven-class system remains the only published attempt to classify clips by thickness, and the Brennholt Institute does not present it as a norm.
The Institute's review panel objected on 3 March 2026 that "thickness" and "wire diameter" should not be used as synonyms, since a plastic-sleeved clip is thicker than its wire by the sleeve, between 0.3 mm and 0.5 mm in the Institute's own sample. Brindlecott's answer is that the sleeve is not part of the clip and that a sleeved clip is classified by its core. The panel's note stands.
The boundary between BR-03 and BR-04, at 0.97 mm to 0.98 mm, remains the Institute's most disputed figure: a correspondent argued in 2025 that the cut falls inside the tolerance of ordinary drawing wire. Brindlecott has not published the histogram that would settle it.