In many radioamateur description coming from the US, wire dimensions are given in american AWG (American Wiring Gauge) standard..
The conversion formula is :
Where mm = diameter in mm and AWG= diameter in AWG
# 0000 AWG=3
# 000 AWG=2
# 00 AWG=1
# 0 AWG=0
Here the table which will help you to convert AWG in mm.
Wikipedia: American Wire Gauge (AWG)
18AWG or AWG18 for PSU power cables / extension cables / modular cables
22AWG or AWG22 for fan power cables / fan extension cables / fan adapter cables