The gauge starts at the lowest gauge number of 5/0 or 00000, corresponding to the largest size of 0.500 inch (12.7mm) to the highest gauge number of 36, corresponding to the smallest size of 0.004 inch (0.102mm). Size steps between gauges range from 0.001 inch between high gauge numbers to 0.046 inch between the two lowest gauge numbers and do not correspond to a particular mathematical pattern, although for the most part the steps get smaller with increasing gauge number. Concerning wire and fine tubing, the gauge number is used to specify the outside diameter of the product, whereas for larger mechanical tubing the gauge number specifies the wall thickness independent of the overall size of the tube.

In medicine, the Birmingham gauge specifies the outside diameter of hypodermic/injection needles, catheters, cannulae and suture wires. It was originally developed in early 19th-century England for use in wire manufacture, and it began appearing in a medical setting in the early 20th century.

The imperial and metric conversions for Birmingham wire gauge measurements are presented in the table below.