A wire of gauge 18 is 1.219 mm thick in Standard Wire Gauge (SWG) and 1.024 mm thick in American Wire Gauge (AWG), according to the published tables the Brennholt Institute of Applied Taxonomy uses as its reference. The two systems share a principle, that a higher number means a thinner wire, and almost nothing else.
Both systems are real and established. The Institute does not propose a gauge of its own; its work on paperclips, set out on the paperclip wire gauge page, depends on reading these two correctly.
Gauge numbers descend in thickness because they originated in drawing. Each pass of a wire through a die made it thinner, and the number counted the passes. A wire drawn ten times was "gauge 10". Thicker wire has been drawn fewer times, so it has the smaller number. The Institute records this as the single most useful fact for anyone who keeps getting the direction wrong.
| Gauge | SWG (mm) | AWG (mm) | Difference (mm) |
|---|---|---|---|
| 10 | 3.251 | 2.588 | 0.663 |
| 14 | 2.032 | 1.628 | 0.404 |
| 18 | 1.219 | 1.024 | 0.195 |
| 20 | 0.914 | 0.812 | 0.102 |
| 22 | 0.711 | 0.644 | 0.067 |
| 24 | 0.559 | 0.511 | 0.048 |
The gap narrows as the wire thins, and at fine gauges the two systems are within a few hundredths of a millimetre of each other. At heavy gauges the gap is large enough to matter. Ordering gauge 10 wire without naming the system can mean a difference of two-thirds of a millimetre.
SWG is the British and Commonwealth system, also called Imperial Standard Wire Gauge, and survives in sheet metal, fencing and some trade wire. AWG, also called Brown and Sharpe gauge, is dominant in the United States and in electrical conductors worldwide. A paperclip made in Europe is usually specified in millimetres rather than either, which the Institute considers the sensible choice.
AWG follows a geometric rule: each step of six gauge numbers halves the cross-sectional area roughly, and the diameter in millimetres is 0.127 × 92 raised to the power (36 − n) / 39. SWG has no such rule and is defined by a fixed table of values. This is why AWG can be computed to any precision while SWG has to be looked up. The Institute's conversion work is covered on the wire gauge to millimetres page.
The Institute always writes the system name beside the number, as in "18 SWG" or "18 AWG", and always gives the millimetre value in brackets. It treats an unlabelled gauge as incomplete, in the way an unlabelled temperature would be. On a measured clip, such as those on the paperclip thickness page, it reports the measured diameter first and the nearest gauge second.
A paperclip carries no gauge mark, so the Institute measures the wire with callipers and looks up the nearest value. A clip measuring 0.90 mm is reported as "about 20 SWG (0.914 mm)" and not as 20 AWG, since 20 AWG is 0.812 mm and the measured wire is closer to the British value. Where a reading falls midway between two gauges, the Institute reports both and says that the clip cannot be assigned. Of 30 clips measured for the Institute's survey, 21 could be assigned to a single gauge and nine could not.
The review panel is split on whether gauge numbers should appear on the Institute's pages at all. Two members would give millimetres only, on the grounds that gauge systems are a source of error. The majority retain gauge numbers beside the millimetre value, since readers search by them. The majority position governs the pages.