Wire gauge calculator
The gauge number counts drawing operations: a wire pulled through twelve dies is 12 gauge, and each pass makes it thinner. That is why a bigger number means a thinner wire, and why the scale is geometric rather than linear — every six gauges roughly halves the cross-section, and every three gauges changes it by about a factor of two in diameter squared.
The ampacity figures are rough free-air estimates for comparison only. Real current ratings depend on insulation type, grouping, ambient temperature and installation method, and are set by the wiring regulations that apply to you.
AWG diameter is 0.127 × 92^((36−n)/39) millimetres, so the scale is geometric and runs backwards — a larger number means a thinner wire. AWG 12 is 3.31 mm², close to the metric 4 mm² but not equal to it.
How to convert wire gauge
AWG and metric sizes never line up exactly, which matters when substituting one for the other. AWG 14 is 2.08 mm² against a metric 2.5; AWG 12 is 3.31 against a metric 4. Going down in size to the nearest AWG below a metric figure means a cable with measurably more resistance and less capacity, so the safe substitution is always the next size up. Stranded wire adds another wrinkle: the AWG number refers to the total conductor cross-section, but a stranded bundle has a larger overall diameter than a solid conductor of the same gauge.
Questions
3.31 mm². The nearest metric size up is 4 mm², which is the safe substitution.