For round enameled copper or aluminium wires, in many countries people like to use Standard Wire Gauge(SWG) or American Wire Gauge(AWG) to define the wire diameters. 

If you want to convert SWG or AWG to mm, you can use the following chart or website.

Website for conversion from SWG to mm:

SWG to mm | Standard gauge to mm conversion (rapidtables.com) 

Website for conversion from AWG to mm:

AWG to mm2 | Gauge to mm conversion (rapidtables.com)