How do I calculate the proper width of a copper trace based on a given gauge (AWG) of wire?
There are formulas around to calculate current handling for various shapes/sizes of wire/trace, so rather than convert just calculate directly. There are various standards around (e.g. IPC2221 - was IPC-D-275 I think).
Rather than memorising formulas from the standards I think many use a tool of some sort. MiscEl is quite a useful little tool, amongst many other things, it has calculations for wires in AWG/mm/in/mil/etc - number of strands or solid core. For traces you put in desired width, temp rise and copper thickness and it will give you the max current for inner/outer traces.