AWG vs metric wire sizing.

The American Wire Gauge in one paragraph

AWG is a US-developed wire sizing system where each "gauge number" corresponds to a specific diameter. The system runs from 4/0 (read "four-aught," the thickest in common use) down to 40 and beyond (the thinnest). Each gauge step is a constant ratio in diameter — about 0.890× per step — which means the area changes by a factor of 2 every three gauge numbers. This logarithmic spacing is the system's main quirk and the source of most confusion.

"Smaller number, bigger wire" — and why

AWG numbers count down as wire gets thicker. 14 AWG is a typical residential branch circuit wire (about 1.6 mm diameter). 8 AWG is much thicker (3.3 mm). 1 AWG is bigger still. After 1 AWG comes 1/0 ("one-aught"), then 2/0, 3/0, 4/0. This is the opposite of metric wire, where the number IS the cross-sectional area in mm² — so a 6 mm² wire is bigger than a 1.5 mm² wire, exactly as you'd expect.

The reason for the inverted numbering is historical. AWG was originally defined by how many times a wire was drawn through a die to make it thinner — each "draw" reduced the diameter. A "20-gauge" wire had been drawn 20 times; a "1-gauge" wire only once. The actual draw-count meaning disappeared long ago, but the inverted numbering survived.

The conversion isn't a simple ratio

Because AWG is logarithmic in diameter and metric mm² is linear in area, the relationship between them is logarithmic, not proportional. There's no constant factor like "1 inch = 2.54 cm." Instead:

d(n) in inches = 0.005 × 92^((36 − n) / 39)

Where n is the AWG number (with 1/0, 2/0, 3/0, 4/0 treated as 0, −1, −2, −3). From the diameter, the metric area in mm² is π × (d/2)². This formula is the official ASTM B258 definition.

The cross-reference you actually need

Memorize these closest matches — they cover 95% of practical work:

Note that the metric sizes don't exactly match AWG — they're "close enough" rather than identical. The metric series rounds to convenient values (1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 mm²). When you're sourcing a 12 AWG cable in a metric country, you'd typically buy 4 mm² — slightly larger than the AWG spec, which is fine, but slightly more expensive.

The ampacity question

Wire size doesn't determine current capacity alone. Insulation type, ambient temperature, conduit fill, and conductor count all matter. In the US, the NEC's Table 310.16 is the authority. In Europe, the equivalent is IEC 60364-5-52. The two standards use different methodology and don't always agree.

A typical example: 2.5 mm² European cable (close to 14 AWG) is rated for 24-27 A in a typical installation per IEC; 14 AWG in the US is rated for 15-20 A per NEC (with §240.4(D) limiting to 15 A regardless of insulation). The European rating isn't "wrong" — it's based on different assumed installation conditions (ambient temperature, bundling, length). Always size to the standard in your jurisdiction.

When you have to source internationally

Three concrete situations come up frequently:

Buying a European appliance for a US installation

The appliance comes with metric cable. If the cable terminates in a plug for the local market, you can usually replace the plug with a US one and proceed. If the cable enters the wall, you need a junction transition: terminate the metric cable in a junction box, splice to AWG using listed connectors, and run AWG to the panel. Most jurisdictions require the AWG portion to comply with NEC.

Buying US wire for export

Round up the metric size. 14 AWG → 2.5 mm² (the AWG is 2.08 mm²; the metric is 2.5 mm², slightly bigger). 12 AWG → 4 mm². 10 AWG → 6 mm². The metric wire will safely handle the same or higher current; the cost increase is modest.

Reading a Chinese or Korean spec sheet

Asian electronics specs are usually metric. A "0.5 mm²" stranded wire in a Chinese spec is about 20 AWG. A "0.75 mm²" wire is about 18 AWG. These tiny gauges are common in flexible cords and appliance cables. Use our AWG cheat sheet for the full cross-reference.

The SWG trap

One more historical wire-sizing system you may run into is SWG — Standard Wire Gauge, originally British, used in some older equipment and still occasionally specified in industries with British heritage (rail, certain types of fasteners, music wire). SWG numbers look similar to AWG numbers but the actual diameters differ. A 16 SWG wire is 1.626 mm; a 16 AWG wire is 1.291 mm. Don't substitute without checking — they're meaningfully different.

Common pitfalls

• Assuming AWG and mm² scale linearly. They don't. Three AWG steps double the area. Six steps quadruple it. Don't try to interpolate "12.5 AWG" by averaging 12 and 13 — the area difference is geometric.
• Confusing stranded and solid AWG. A stranded conductor has the same nominal AWG as a solid one, but the actual copper cross-section is 2-5% less (because of the air gaps between strands). For high-precision work, this matters.
• Forgetting the small-conductor rule. NEC §240.4(D) limits the overcurrent protection of 14, 12, and 10 AWG copper to 15, 20, and 30 A respectively — even though their 75°C and 90°C ampacities are higher. See our wire gauge calculator which handles this automatically.
• Mixing aluminum and copper sizing. Aluminum wire has about 60% the conductivity of copper. A given current needs a larger aluminum wire than copper — typically one full AWG larger. NEC tables give separate aluminum ampacity values.

Sources & further reading

• AWG diameter formula: ASTM B258 — Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires Used as Electrical Conductors.
• Metric wire sizes: IEC 60228 — Conductors of insulated cables. Specifies the metric series (0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 mm²) and the maximum DC resistance per km.
• Ampacity (US): NFPA 70 (NEC), Table 310.16.
• Ampacity (international): IEC 60364-5-52 — Electrical installations of buildings — Selection and erection of electrical equipment — Wiring systems.

Related on unitcheatsheet.com