Wikipedia has a precise AWG formula:

http://en.wikipedia.org/wiki/American_wire_gauge
Unfortunately, it's arranged to convert AWG number to diameter. The

original poster asked for the opposite conversion. Rearranging the

Wikipedia formula to run that way gives:

39 × log (200 × d)

AWG = 36 - --------------------

log 92

where d = diameter in inches.

The equation looks gnarly, but it's mostly constants and can

be simplified to this approximation:

AWG = -9.7 - 19.86 × log d

That's accurate within ±.01 AWG, from 0000 to 50 gauge.