🌐AWG to mm²

By the AmpSizer editorial desk

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Multiply the area in thousands of circular mils by 0.5067 to get square millimetres. 12 AWG is 6.53 kcmil, which is 3.31 mm²; 4/0 is 211.6 kcmil, which is 107.2 mm².

The two scales never land on each other. 3.31 mm² sits between the stocked metric sizes of 2.5 and 4 mm², so a conversion is a reading and not a substitution — and going down from 12 AWG to 2.5 mm² is a 24% loss of copper.

The safe direction is upward. 2.5 mm² is between 14 and 12 AWG, 4 mm² is just above 12, 6 mm² is just above 10, 10 mm² is just above 8, 16 mm² is just above 6. Round to the larger member of the pair and the ampacity looks after itself.

The conversion table

Every size, both scales, with the diameter and the insulated area that conduit fill is worked out from.

Conductor size, area, resistance and allowable ampacity. Ampacity is NEC Table 310.16 — not more than three current-carrying conductors in a raceway, cable or earth, at 86 °F (30 °C) ambient. Correct it for heat and for bundling before you use it.
Size	kcmil	mm²	Diameter, in	THHN area, in²
14 AWG	4.11	2.08	0.073	0.0097
12 AWG	6.53	3.31	0.092	0.0133
10 AWG	10.38	5.261	0.116	0.0211
8 AWG	16.51	8.367	0.146	0.0366
6 AWG	26.24	13.3	0.184	0.0507
4 AWG	41.74	21.15	0.232	0.0824
3 AWG	52.62	26.67	0.26	0.0973
2 AWG	66.36	33.62	0.292	0.1158
1 AWG	83.69	42.41	0.332	0.1562
1/0 AWG	105.6	53.49	0.373	0.1855
2/0 AWG	133.1	67.43	0.419	0.2223
3/0 AWG	167.8	85.01	0.47	0.2679
4/0 AWG	211.6	107.2	0.528	0.3237
250 kcmil	250	127	0.575	0.397
300 kcmil	300	152	0.63	0.4608
350 kcmil	350	177	0.681	0.5242
400 kcmil	400	203	0.728	0.5863
500 kcmil	500	253	0.813	0.7073
600 kcmil	600	304	0.893	0.8676
700 kcmil	700	355	0.964	0.9887
750 kcmil	750	380	0.998	1.0496

Nearest stocked metric size

The two systems interleave rather than align. This is where each common gauge falls between the metric sizes that are actually sold, and which one to buy for a like-for-like replacement.

AWG	Exact mm²	Between	Use
14 AWG	2.08 mm²	1.5 and 2.5	2.5 mm²
12 AWG	3.31 mm²	2.5 and 4	4 mm²
10 AWG	5.26 mm²	4 and 6	6 mm²
8 AWG	8.37 mm²	6 and 10	10 mm²
6 AWG	13.3 mm²	10 and 16	16 mm²
4 AWG	21.2 mm²	16 and 25	25 mm²
2 AWG	33.6 mm²	25 and 35	35 mm²
1/0 AWG	53.5 mm²	50 and 70	70 mm²
2/0 AWG	67.4 mm²	50 and 70	70 mm²
4/0 AWG	107.2 mm²	95 and 120	120 mm²
250 kcmil	127 mm²	120 and 150	150 mm²
500 kcmil	253 mm²	240 and 300	300 mm²

Round up, always. The pair either side of a gauge is typically 30 to 40% apart in area, so choosing the lower one is a real loss of ampacity and a real increase in voltage drop — not a rounding error.

The other units, and why they all have a switch

Wire size is not the only figure that means nothing to half the world. Horsepower, Fahrenheit, feet and cubic inches are all in the same position, so every quantity on this site carries both names and a switch beside the field:

Quantity	US, the default	Metric	Exactly
Conductor size	AWG, kcmil	mm²	1 kcmil = 0.5067 mm²
Run length	feet	metres	1 ft = 0.3048 m
Ambient temperature	°F	°C	°C = (°F − 32) × 5/9
Motor rating	horsepower	kilowatts	1 HP = 0.7457 kW
Box volume	cubic inches	cm³	1 cu in = 16.387 cm³
Tray width, diameters	inches	millimetres	1 in = 25.4 mm

Volts, amps, watts and ohms need no switch: they are SI units already, and they are the same everywhere.

The horsepower trap

Mechanical horsepower is 745.7 W. Metric horsepower — PS in German, CV in French, Spanish and Italian — is 735.5 W. They differ by 1.4%.

A US motor nameplate means the first. An imported machine may mean the second, and when a “5 PS” motor is read as 5 HP the current comes out 1.4% low. Most of the time that vanishes into the code table’s own rounding. At the boundary between two rows of Table 430.248 it does not, and the conductor comes out one size short.

Our motor calculator prints both, so the nameplate can be checked against the right one.

Why the NEC answer is not a European answer

The arithmetic on this site is unit-agnostic — a volt is a volt — but the tables are not. NEC Table 310.16 assumes US installation methods, US temperature bands and US conductor insulations. IEC 60364 organises the same question by installation method, and its figures for a given cross-section are different, sometimes substantially.

So the metric columns here are for reading and for buying cable, not for compliance. Calculations follow the US NEC; outside the US, follow your local electrical code.

Frequently asked questions

What is 12 AWG in mm²?

3.31 mm² exactly, which is between the stocked 2.5 and 4 mm² metric sizes. For a like-for-like replacement use 4 mm²; 2.5 mm² is 24% less copper and a corresponding rise in voltage drop. The same relationship holds all the way up the scale, because the two systems were never harmonised.

What does AWG stand for and why do the numbers run backwards?

American Wire Gauge, and the numbers count drawing operations: wire was pulled through progressively smaller dies and the gauge recorded how many passes it took. More passes means thinner wire and a higher number. It also makes the scale geometric, which is why every three gauge numbers is almost exactly a doubling of area.

Can I use an NEC ampacity for a metric conductor?

Not as a compliance answer. NEC Table 310.16 is a US table used with US installation methods and US temperature bands; IEC 60364 organises ampacity by installation method and gives different figures for the same cross-section. The mm² column here is a conversion for reading and buying, not a claim that an NEC result satisfies another country’s code.

Is horsepower the same everywhere?

No, and it is the unit that catches people hardest. Mechanical horsepower — what a US motor nameplate means — is 745.7 W. Metric horsepower, written PS or CV, is 735.5 W. A motor labelled 5 PS is 4.93 mechanical HP, and at the boundary of a code table that 1.4% is occasionally the difference between two rows.

Which units does this site let me switch?

All of them. Length between feet and metres, temperature between Fahrenheit and Celsius, conductor size between AWG and mm², motor rating between horsepower and kilowatts, box volume between cubic inches and cubic centimetres, and tray width between inches and millimetres. The defaults are US because the tables are, and the switch sits next to the field.

How we calculate this

Area in square millimetres is the circular-mil area multiplied by 0.50670748 — one circular mil is the area of a circle one thousandth of an inch across, so a thousand of them is 0.5067 mm². Both columns are taken directly from NEC Chapter 9, Table 8 rather than recomputed, so they match the code book to its own rounding. Nearest metric equivalents are given as the pair of stocked IEC sizes a gauge falls between, with the larger one recommended, because rounding down loses ampacity and adds voltage drop. Unit conversions used elsewhere on this site: 1 ft = 0.3048 m, 1 in = 25.4 mm, °C = (°F − 32) × 5/9, 1 cu in = 16.387 cm³, 1 mechanical horsepower = 745.7 W and 1 metric horsepower (PS, CV) = 735.5 W. Calculations follow the US NEC; outside the US, follow your local electrical code.

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