How the conversion works
An AWG number stands for a diameter. ASTM B258 prescribes the nominal diameters; as NBS Handbook 100 describes them, each is d = 0.005 in x 92(36 - n)/39, rounded to 0.0001 in. The area in mm² follows from the diameter in mm:
mm² = 3.1416 / 4 x d², with d in mm.
For 12 AWG, d = 0.0808 in = 2.0523 mm, so the area is 0.7854 x 2.0523² = 3.308 mm². From circular mils it is one multiplication: 1 circular mil = 0.0005067 mm², so 6,529 cmil x 0.0005067 = 3.308 mm². For kcmil sizes, mm² = kcmil x 0.5067: 250 kcmil is 126.7 mm².
Diameter in mm and area in mm² are easy to mix up. A wire 2 mm in diameter is close to 12 AWG (2.05 mm, 3.31 mm²). A 2 mm² wire is close to 14 AWG (2.08 mm²), which is only 1.63 mm across.
Metric sizes are a different series
Metric cable follows IEC 60228, which specifies nominal cross-sections from 0.5 mm² to 3,500 mm² for the conductors of power cables and cords. Lapp's technical table, which follows DIN EN 60228, lists the sizes from 0.5 mm² up as 0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 and 300 mm², with larger sizes beyond. None of these equals an AWG area in the chart, so a conversion always lands between two metric sizes.
IEC 60228 also includes resistance values, not just areas. Lapp's table lists 7.41 ohms per km at 20°C as the maximum for a plain copper 2.5 mm² stranded (class 2) conductor. A true 2.5 mm² of annealed copper at the international standard in NBS Handbook 100 would measure 6.90 ohms per km. The limit leaves room for a conductor that carries less effective copper than its name, so compare resistance as well as area when the difference matters.
AWG to the nearest metric size
| AWG | Area (mm²) | Next smaller IEC | Next larger IEC |
|---|---|---|---|
| 20 | 0.519 | 0.5 | 0.75 |
| 18 | 0.823 | 0.75 | 1 |
| 16 | 1.31 | 1 | 1.5 |
| 14 | 2.08 | 1.5 | 2.5 |
| 12 | 3.31 | 2.5 | 4 |
| 10 | 5.26 | 4 | 6 |
| 8 | 8.37 | 6 | 10 |
| 6 | 13.3 | 10 | 16 |
| 4 | 21.2 | 16 | 25 |
| 2 | 33.6 | 25 | 35 |
| 1/0 | 53.5 | 50 | 70 |
| 2/0 | 67.4 | 50 | 70 |
| 3/0 | 85.0 | 70 | 95 |
| 4/0 | 107.2 | 95 | 120 |
| 250 kcmil | 126.7 | 120 | 150 |
| 500 kcmil | 253.4 | 240 | 300 |
Metric size to an AWG size that covers it
When a manual or drawing calls for a metric size, an AWG wire with at least that area has no more resistance in the same metal, because resistance falls in proportion to area:
| IEC size (mm²) | AWG with at least that area |
|---|---|
| 0.75 | 18 AWG (0.823 mm²) |
| 1 | 16 AWG (1.31 mm²) |
| 1.5 | 14 AWG (2.08 mm²) |
| 2.5 | 12 AWG (3.31 mm²) |
| 4 | 10 AWG (5.26 mm²) |
| 6 | 8 AWG (8.37 mm²) |
| 10 | 6 AWG (13.3 mm²) |
| 16 | 4 AWG (21.2 mm²) |
| 25 | 3 AWG (26.7 mm²) |
| 35 | 1 AWG (42.4 mm²) |
| 50 | 1/0 (53.5 mm²) |
| 70 | 3/0 (85.0 mm²) |
| 95 | 4/0 (107.2 mm²) |
| 120 | 250 kcmil (126.7 mm²) |
| 150 | 300 kcmil (152.0 mm²) |
| 185 | 400 kcmil (202.7 mm²) |
| 240 | 500 kcmil (253.4 mm²) |
Through 4 AWG the table gives the even gauge. The odd gauge one step smaller also covers each of those sizes: 17 AWG (1.04 mm²) for 1 mm², 15 AWG (1.65 mm²) for 1.5, 13 AWG (2.63 mm²) for 2.5, 11 AWG (4.17 mm²) for 4, 9 AWG (6.63 mm²) for 6, 7 AWG (10.55 mm²) for 10 and 5 AWG (16.77 mm²) for 16. A 2/0 conductor (67.4 mm²) is just under 70 mm², which is why 70 maps to 3/0.
Before you substitute
- Check that the terminals accept the conductor. The terminal or equipment instructions give the sizes they take.
- Area is not ampacity. The current a wire may carry is not on this page; get it from the electrical code adopted where you work and the cable maker's data. The wire gauge chart explains why it is not listed here.
- For long runs, check the drop with the voltage drop calculator. A larger size has proportionally lower resistance, so it cuts the drop in proportion to the added area.