The types of wire gauge are classed by the system of gauge they measure. There are gauges that use the British standard wire gauge system and ones that use the American wire gauge system.
The two gauge systems are different and cannot be used in conjunction with each other. For example SWG 14 (2.03mm) is closer to AWG 12 (2.05mm) than AWG 14 (1.63mm). The table below shows how the two correlate:
British standard wire gauge system (SWG)
The gauge set to the British standard wire gauge system is used in the UK to measure steel wire. You can tell the difference between this and a gauge set to the American wire gauge system as it has metric measurements on the reverse side. It also starts its measurements from 1, rather than 0.
A gauge for the British standard wire gauge system is available either in a circular or rectangular format.
For more information, see What is a British standard wire gauge?
American Wire Gauge system (AWG)
The American wire gauge system is used to measure American wiring systems, so is only found occasionally in Britain where American wiring has been imported. The gauge set to this system is only found in a circular format and has imperial measurements on the reverse.
For more information, see What is an American wire gauge?