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American Wire Gauge is logarithmic and inverted: diameter is 0.127 x 92^((36-n)/39) mm, so larger numbers mean thinner wire and every six gauges halves the area. AWG 12 is 2.05 mm diameter and 3.31 mm2, giving about 0.156 ohms over 30 m of copper and a continuous rating near 20 A.
AWG diameter
d(mm) = 0.127 x 92^((36 - n) / 39)
Resistance
R = 0.0172 x length / area, for copper at 20 C
Because it counts drawing operations - each pass through a smaller die raised the gauge number and reduced the diameter.
It is a rough 6 A per mm2 guide. Real ampacity depends on insulation type, grouping, ambient temperature and installation method, all set by wiring regulations.