Needle gauge system

Last revised by Daniel J Bell on 19 Jul 2022

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.

Although there is not an equal increment between each step in the system, the caliber decreases by ≈11% per increment. 

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 1.

There is a lack of consistency across the medical literature and more widely about how the gauge should be written 1-4. Using the full form it is straightforward, "1 Gauge" and it is generally capitalized. 

However when abbreviated, it is always capitalized, but could be written as "1 G" or "1G". The former seems more common and would square with official practice for other scientific units 1-4, i.e. a space is left between the numeric quantity and the unit symbol, e.g. for distance, we write "1 cm" and not "1cm". The only common exceptions are for the Celsius temperature scale (1°C not 1 °C) and angle measurements (90° not 90 °).

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 counterintuitive from a modern scientific perspective 1.

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. 

Hypodermic needles follow the International Organization for Standardization (ISO) color coding system for identification 5, with a selection below from larger to smaller:

  • 16 G: white
  • 18 G: pink
  • 20 G: yellow
  • 21 G: deep green
  • 22 G: black
  • 25 G: orange
  • 26 G: brown

Although peripheral and central venous catheters use the same measurement for cannula size, the color system for PVCs is different from the ISO needles above:

  • 14 G: orange
  • 16 G: grey
  • 18 G: green
  • 20 G: pink
  • 22 G: blue
  • 24 G: yellow

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