Birmingham gauge

This progression does not follow a mathematical formula but instead reflects historical manufacturing practices in the wire drawing industry, where the size increments were determined by practical tooling, such as a draw plate, and production constraints.

[4] In applications involving wires and fine tubing, the gauge number refers to the product's outside diameter.

In medicine, the Birmingham gauge is widely used to specify the outer diameter of hypodermic needles, catheters, cannulae, and suture wires.

Hypodermic needles are available in a wide variety of outer diameters described by gauge numbers.

The following chart shows nominal inner diameter and wall thickness for regular-wall needles.

Six hypodermic needles on Luer connectors . These needles are normally used with other medical devices, such as a syringe; from top to bottom:
  • 26G × 1 ⁄ 2 ″ (0.45 × 12 mm) (brown)
  • 25G × 5 ⁄ 8 ″ (0.5 × 16 mm) (orange)
  • 22G × 1 + 1 ⁄ 4 ″ (0.7 × 30 mm) (black)
  • 21G × 1 + 1 ⁄ 2 ″ (0.8 × 40 mm) (green)
  • 20G × 1 + 1 ⁄ 2 ″ (0.9 × 40 mm) (yellow)
  • 19G × 1 + 1 ⁄ 2 ″ (1.1 × 40 mm) (cream)