Two systems, two logics
American Wire Gauge (AWG) is a geometric series. Each gauge number corresponds to a solid wire diameter, and the diameters follow a fixed formula: 36 AWG is 0.127 mm and 0000 (4/0) AWG is 11.68 mm, with 39 equal ratio steps between them. In practice that means every 3 gauge steps roughly doubles or halves the cross-section, and every 6 steps roughly doubles or halves the diameter. Above 4/0, US sizes switch to kcmil (thousands of circular mils), where 1 kcmil is about 0.5067 mm2.
IEC metric sizes, defined in IEC 60228, are a list of nominal cross-sections: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 mm2 and upwards. As explained in our guide to IEC 60228 conductor classes, these are names tied to a maximum DC resistance, not exact geometric areas. AWG sizes, by contrast, are defined by geometry. The two series were built separately and do not share any size.
The conversion table
The table below gives the standard AWG geometry for solid conductors, the resulting cross-section, and the nearest metric sizes either side. Stranded AWG conductors have the same nominal area but a larger overall diameter.
| AWG / kcmil | Solid diameter, mm | Area, mm2 | Next metric size down | Next metric size up |
|---|---|---|---|---|
| 24 AWG | 0.511 | 0.205 | - | - |
| 23 AWG | 0.573 | 0.258 | - | - |
| 18 AWG | 1.024 | 0.823 | 0.75 mm2 | 1 mm2 |
| 16 AWG | 1.291 | 1.31 | 1 mm2 | 1.5 mm2 |
| 14 AWG | 1.628 | 2.08 | 1.5 mm2 | 2.5 mm2 |
| 12 AWG | 2.053 | 3.31 | 2.5 mm2 | 4 mm2 |
| 10 AWG | 2.588 | 5.26 | 4 mm2 | 6 mm2 |
| 8 AWG | 3.264 | 8.37 | 6 mm2 | 10 mm2 |
| 6 AWG | 4.115 | 13.3 | 10 mm2 | 16 mm2 |
| 4 AWG | 5.189 | 21.2 | 16 mm2 | 25 mm2 |
| 2 AWG | 6.544 | 33.6 | 25 mm2 | 35 mm2 |
| 1/0 AWG | 8.252 | 53.5 | 50 mm2 | 70 mm2 |
| 4/0 AWG | 11.68 | 107 | 95 mm2 | 120 mm2 |
| 250 kcmil | - | 127 | 120 mm2 | 150 mm2 |
| 500 kcmil | - | 253 | 240 mm2 | 300 mm2 |
Values are calculated from the AWG formula and rounded. They are reference geometry, not measured product values; the datasheet for a specific construction gives its actual conductor diameter and resistance.
Be careful with diameter as a shortcut. A stranded conductor of the same gauge is wider than the solid figure because of the gaps between wires, and a compacted one is narrower again. THHN 12 AWG is a good example: per a large maker's datasheet the 19-strand version is about 0.130 in overall, against about 0.122 in for solid. Never identify a size from a caliper reading of the cable.
Which direction is safe
The rule that prevents most mistakes is simple: when converting a design size, go to the next size up, never the nearest size. A design calling for 12 AWG (3.31 mm2) is not met by 2.5 mm2 cable, even though 2.5 mm2 is a common size and only one step away. It is met by 4 mm2, subject to the project engineer confirming the other checks. Going the other way, a European design calling for 2.5 mm2 is not met by 14 AWG (2.08 mm2); it is met by 12 AWG.
The reason is that the circuit was designed around a resistance and a current rating for the original size. A smaller conductor has higher resistance, more voltage drop and more heating. A larger one costs more copper and may not fit the terminals, but it is electrically on the safe side. The voltage drop article shows how resistance turns into volts lost on a long run, and the cable sizing guide explains the full method. Current ratings themselves depend on the installation method and must come from IEC 60364-5-52, BS 7671 or the NEC as applicable, confirmed by the project engineer.
Conversion is not compliance
Even a correct conversion does not make a metric cable acceptable on a US job, or an AWG cable acceptable on an IEC job. US projects usually require UL (or other NRTL) listed products to satisfy the AHJ, and those listings are for AWG sizes under standards such as UL 83 for THHN/THWN-2 and UL 44 for XHHW-2. A 4 mm2 cable to IEC 60227 is not a listed 12 AWG product, however close the numbers. In the other direction, IEC and EN projects expect metric sizes verified against IEC 60228 resistance limits.
NEC 310.3 adds a floor in the US: the minimum building conductor is 14 AWG copper or 12 AWG aluminium and copper-clad aluminium. A buyer importing metric cable for a US market cannot simply pick an equivalent size; the product must be the AWG construction the market expects. The building wire page lists the US and IEC constructions partner plants supply, including THHN 12 AWG, whose solid version has an OD of about 0.122 in per a large maker's datasheet.
Small gauges in data cable
AWG also appears in network cable, where sizes are small and conversion matters less than material. The reference Cat6 U/UTP construction uses 23 AWG solid copper conductors of about 0.555 to 0.58 mm diameter per public datasheets, close to the 0.573 mm geometry in the table. Patch cords and some Cat5e use 24 AWG or 26 AWG. Here the point to check is not the metric equivalent but that the conductor is solid copper, because category cable requires solid copper conductors; the CCA vs copper article explains how substitutes are spotted.
A checklist for mixed-standard projects
- Keep the original unit: copy the size exactly as the design states it, in AWG, kcmil or mm2.
- Convert only with engineering sign-off: if a size must change system, round up and ask the project engineer to confirm current rating, voltage drop and short-circuit checks.
- Match the standard to the unit: AWG sizes with UL and NEC constructions; metric sizes with IEC, EN or BS constructions.
- Check terminals: a larger metric size may not fit a breaker or lug designed for the AWG size.
- Verify by resistance: ask for DC resistance records per drum against the limit for the stated size and standard.
When you send an enquiry through the quote form, give sizes in the unit your design uses and name the target market. The desk will flag any mismatch between size, standard and market before partner plants quote.