For some reason, I had never bothered to pursue the relationship between string gauge and actual string diameter. However, when I sorted my stash of old strings after putting brand new Elixir medium strings in order to sound best at next week's concert, I found several strings for which I didn't know the gauge, meaning they would be next to useless in case of an emergency (which is typically the only situation under which I'd put on a used string). And I only had a metric vernier caliper available...
Of course, since the gauge does not directly translate to a measurement in metric units, I had heretofore assumed the translation between gauge and diameter to be as esoteric as the one for the American Wire Gauge and skipped the issue.
However, it turned out to be much simpler than with standard wires: the string gauge is simply the diameter in inches. So the conversion is ultra-simple: taking my favorite D string at 0.065 gauge (sometimes noted 65), multiply it by 25.4 to obtain the string metric diameter: 1.65mm.
Conversely, what's the gauge of that bit fat low C string I use on my CGDA-tuned Blazer ? Hmmm... :
3.2mm → 3.2/25.4 = 0.126.
So it's a [0.]125 gauge: string manufacturers only supply strings with gauges rounded to a multiple of .005. That's all there's to it ! Now you no longer need to guess. Just dig out that old slide caliper from technology class, and label your spare strings !
Of course, if you're using one of these shiny new digital micrometers, chances are you won't even have to do the calculation, since these tend to be switchable from international standard units to the old imperial units.