Needle gauge system

The needle gauge system, often called just the Gauge or G, is an internationally-used scale for sizing needles. It was adopted from pre-existing gauges which were used in industry to size metal wire.

In contradistinction to the French scale, the other well-known sizing system, which is a metric system, the Gauge size cannot be derived from a straightforward formula. This is because the size was not designed to be an absolute measure, but instead a relative scale. Moreover, as the Gauge number increases, the needle caliber decreases.

The lowest and highest allowable gauges are pre-defined and published tables state the sizing of every available gauge. Attempts have been made to replace the gauge system with a logical, modern system, but have gained no traction until now .

Gauge sizing

The history of the gauge is surprisingly long and complex, and outside the scope of this article. However, an appreciation of it is the best way to understand why the scale appears so counter-intuitive from a modern scientific perspective .

The gauge originally arose as a method for measuring the thickness of metal wire, in particular, iron wire. For centuries, wire was manufactured by a process known as 'drawing'. The process of wire-making started with a thick wire, and thinner wires were made by pulling (or drawing) the wire through progressively smaller holes. The size of the largest wire that one could start with was limited by the force that could be generated to pull it through a smaller hole. In the pre-steam age, the greatest force could be supplied by horse or a water-wheel.

Originally the largest wire was given the designation of 1 G, and then for each successively smaller caliber, the gauge number was increased by one. Therefore as the wires get smaller, the gauge number increases. However, as later technology (e.g. steam power) permitted larger wires to be used as the starting needle in the drawing process, the scale had to be extended downwards. To avoid negative numbers the gauges greater than 1 were designated 0, 1/0, 2/0 etc.

Each wire-maker would have a standard metal gauge for their customers' use by which they could choose the thickness of wire they might require and also verify that their purchased wire was of the stated size. This system did not require any absolute measurement of the wire thickness. Indeed in the pre-industrial era, technology was not yet sophisticated enough to perform accurate, consistent measuring. This explains why the system is comparative and not absolute.

When the UK became the first country in the world to standardize a gauge system in 1884, an attempt was made to make the increments between the gauge numbers more regular. This system was known as the British Standard Wire Gauge, and it was developed from the older Birmingham gauge system.  Although there is not an equal increment between each step in the system, the caliber decreases by ≈11% per increment.