18 awg wire = 1 mm diameter wire (almost exactly)

1 mm átmérőjű vezeték = 1mm diameter wire

18 AWG = 1.02362 mm, so "1 mm diameter wire" is good.

Please see the conversion table from American Wire Gauge into mm at the address provided below.
(You will need to scroll down)


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Note added at 25 mins (2005-06-30 02:36:06 GMT)
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If you want to be absolutely correct, you could write this:
1 mm átmérőjű (18 AWG) vezeték
AWG is used and understood among professionals, and when they look at the metric size, sometimes it is not the diameter they pay attention to, but the area which is measured in mm2 (square millimiter). 18 AWG is approximately 0.8mm2.