AWG to mm² conversion
AWG numbers fall as the wire gets thicker, and every three gauges roughly doubles the cross-section. Metric cable is sold by cross-section in mm². Where an AWG size falls between two metric sizes, use the next metric size up unless you have checked the drop and current for the smaller one.
Use 8 AWG (8.37 mm²) copper or larger.
Voltage drop sets the size. 10 AWG would drop 3.33%, over your 3% limit. Ampacity alone would allow 14 AWG.
Check: 8 AWG (8.37 mm²)
| Load current | 20 A |
| Wire in the circuit (out and back) | 2 x 10 ft = 20 ft |
| Resistance of 8 AWG | 0.6282 ohm per 1000 ft; 0.01256 ohm for this circuit |
| Voltage drop | 0.251 V = 2.09% within 3% |
| Voltage at the load | 11.75 V |
| Power lost in the wire | 5.03 W |
| Ampacity of 8 AWG | 50 A (NEC 310.16, 75 C) ok |
Vd = I x 2L x R / 1000 = 20 x 2 x 10 x 0.6282 / 1000 = 0.251 V, and 0.251 / 12 = 2.09%. Length L in feet, R in ohms per 1000 ft.
| AWG | mm² | Drop (V) | Drop (%) | At load (V) | Rated (A) | Result |
|---|---|---|---|---|---|---|
| 18 | 0.82 | 2.554 | 21.28 | 9.45 | 10* | drop and current |
| 16 | 1.31 | 1.606 | 13.39 | 10.39 | 13* | drop and current |
| 14 | 2.08 | 1.01 | 8.42 | 10.99 | 20 | too much drop |
| 12 | 3.31 | 0.635 | 5.29 | 11.36 | 25 | too much drop |
| 10 | 5.26 | 0.4 | 3.33 | 11.6 | 35 | too much drop |
| 8 | 8.37 | 0.251 | 2.09 | 11.75 | 50 | smallest that passes |
| 6 | 13.3 | 0.158 | 1.32 | 11.84 | 65 | passes |
| 4 | 21.2 | 0.099 | 0.83 | 11.9 | 85 | passes |
| 3 | 26.7 | 0.079 | 0.66 | 11.92 | 100 | passes |
| 2 | 33.6 | 0.063 | 0.52 | 11.94 | 115 | passes |
| 1 | 42.4 | 0.05 | 0.41 | 11.95 | 130 | passes |
| 1/0 | 53.5 | 0.039 | 0.33 | 11.96 | 150 | passes |
| 2/0 | 67.4 | 0.031 | 0.26 | 11.97 | 175 | passes |
| 3/0 | 85 | 0.025 | 0.21 | 11.98 | 200 | passes |
| 4/0 | 107 | 0.02 | 0.16 | 11.98 | 230 | passes |
* chassis-wiring guidance; 14 AWG and up: NEC Table 310.16, copper, 75 C column.
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AWG to mm² table
| AWG | mm² | Solid diameter (mm) | Nearest metric size | Next metric size up | ohm per km |
|---|---|---|---|---|---|
| 18 | 0.82 | 1.02 | 0.75 mm² | 1 mm² | 20.9482 |
| 16 | 1.31 | 1.29 | 1.5 mm² | 1.5 mm² | 13.1759 |
| 14 | 2.08 | 1.63 | 2.5 mm² | 2.5 mm² | 8.2841 |
| 12 | 3.31 | 2.05 | 4 mm² | 4 mm² | 5.21 |
| 10 | 5.26 | 2.59 | 6 mm² | 6 mm² | 3.2772 |
| 8 | 8.37 | 3.26 | 10 mm² | 10 mm² | 2.061 |
| 6 | 13.3 | 4.12 | 16 mm² | 16 mm² | 1.2963 |
| 4 | 21.2 | 5.19 | 25 mm² | 25 mm² | 0.8153 |
| 3 | 26.7 | 5.83 | 25 mm² | 35 mm² | 0.6463 |
| 2 | 33.6 | 6.54 | 35 mm² | 35 mm² | 0.5128 |
| 1 | 42.4 | 7.35 | 35 mm² | 50 mm² | 0.4065 |
| 1/0 | 53.5 | 8.25 | 50 mm² | 70 mm² | 0.3224 |
| 2/0 | 67.4 | 9.27 | 70 mm² | 70 mm² | 0.2557 |
| 3/0 | 85 | 10.4 | 95 mm² | 95 mm² | 0.2028 |
| 4/0 | 107 | 11.68 | 95 mm² | 120 mm² | 0.1608 |
Diameter of a solid conductor from the AWG formula; stranded cable is larger overall. Metric sizes are the common IEC 60228 series.
Worked example
20 A at 12 V, 10 ft one way, 3% drop allowed. The current goes out and comes back,
so the circuit is 2 x 10 = 20 ft of wire, and the allowed drop is 12 x 3 / 100 =
0.36 V. Voltage drop is Vd = I x 2L x R / 1000, with R in ohms per 1000 ft:
- 10 AWG:
20 x 20 x 0.9989 / 1000 = 0.4 V, which is 3.33% of 12 V; over the 3% limit. - 8 AWG:
20 x 20 x 0.6282 / 1000 = 0.251 V, which is 2.09% of 12 V; passes both checks.
Answer: 8 AWG (8.37 mm²), rated 50 A, the smallest size that passes both.
24V quick chart at 3% drop
| Amps | 5 ft 1.5 m | 10 ft 3 m | 20 ft 6.1 m | 30 ft 9.1 m | 50 ft 15.2 m |
|---|---|---|---|---|---|
| 5 A | 18 | 18 | 14 | 12 | 10 |
| 10 A | 18 | 14 | 12 | 10 | 8 |
| 20 A | 14 | 12 | 8 | 6 | 4 |
| 30 A | 10* | 10 | 6 | 6 | 3 |
| 50 A | 8* | 8 | 4 | 3 | 1 |
| 100 A | 3* | 3* | 2 | 1/0 | 3/0 |
Copper, minimum AWG. * ampacity, not voltage drop, sets this size. "over 4/0" means no single copper conductor up to 4/0 passes: shorten the run, use a higher system voltage or run conductors in parallel. Length is one way; the math uses twice that, out and back.
Before you buy wire
- Ampacity figures for 14 AWG and larger are from NEC (NFPA 70) Table 310.16, copper, 75 C column, which assumes 30 C ambient and no more than three current-carrying conductors bundled together. Hot spaces, bundles and conduit lower them. 18 and 16 AWG figures are chassis-wiring guidance, not NEC ratings.
- On boats, follow ABYC E-11 for DC wiring. Engine rooms are hot, and conductors there are allowed less current than the same wire elsewhere.
- Use wire rated for the location: heat, oil, water, sunlight and abrasion all matter. Stranded copper is the norm in vehicles and boats, where wire vibrates.
- Fuse every positive run close to the source, sized to protect the wire. NEC 240.4(D) limits the overcurrent device on 14, 12 and 10 AWG to 15, 20 and 30 A.
- This calculator does arithmetic. It does not certify an installation; check your work against the standards and the equipment makers' instructions, or have it checked.
Questions
Is 6 mm² the same as 10 AWG?
Close. 10 AWG is 5.26 mm², so 6 mm² is slightly larger and carries the same current with a little less drop.