There are 2 gauge systems AWG (american wire gauge) and SWG (standard wire gauge). 16g awg is 18g swg, so 1.2mm x 9/32" ID.
We use SWG.
10AWG = 12SWG = 0.1" = 2.54mm
12AWG = 14 SWG = 0.08" = 2mm
14AWG = 16SWG = 0.063" = 1.6mm
16AWG= 18SWG = 0.048" = 1.2mm
18AWG=19SWG = 0.04" = 1mm
At 20g you will not find a difference as our gauges are AWG. Confusing, I know.
20AWG = our size = 0.032" = 0.8"
We do list all of our sizes in decimal inches and mm on our site as well as gauges.
I highly recommend shopping in mm.
You may also find this chart useful: http://theringlord.com/cart/shopcontent.asp?type=WireGauge