How the AWG scale works
American Wire Gauge is a logarithmic scale defined from two points: 36 AWG is 0.005 in and 0000 (4/0) is 0.46 in, with 38 equal ratio steps in between. The diameter of any gauge n is:
d (in) = 0.005 × 92(36 − n) ÷ 39
For 1/0 use n = 0, 2/0 is n = −1, 3/0 is n = −2 and 4/0 is n = −3. Area in circular mils is diameter in mils squared: a 0.1019 in (101.9 mil) 10 AWG conductor is 101.9² ≈ 10,380 cmil. Convert to mm² by multiplying by 0.0005067.
Rules of thumb
- Every 3 gauges smaller doubles resistance and halves area. 12 AWG (1.93 Ω/kft) has almost twice the resistance of 9 AWG.
- Every 6 gauges halves the diameter.
- 10 AWG copper is about 1 Ω per 1000 ft (1.21 Ω at 75 °C), a handy reference point.
- Aluminum has about 61% of copper's conductivity, so aluminum is usually sized two gauges larger for the same ampacity.
Example: comparing two sizes
Going from 12 AWG to 10 AWG raises area from 6,530 to 10,380 cmil, a 59% increase. Resistance falls from 1.93 to 1.21 Ω/kft, so voltage drop on the same run falls by 37%. On a 100 ft, 16 A, 120 V circuit that means 3.23% instead of 5.15%. Try it in the voltage drop calculator.
AWG vs metric sizes
Canada uses AWG for building wire, but most of the world sizes conductors in mm². Approximate equivalents: 14 AWG ≈ 2.08 mm² (next IEC size 2.5 mm²), 12 AWG ≈ 3.31 mm² (4 mm²), 10 AWG ≈ 5.26 mm² (6 mm²), 8 AWG ≈ 8.37 mm² (10 mm²), 6 AWG ≈ 13.3 mm² (16 mm²). Never substitute a smaller metric size for an AWG size.
Choosing a gauge
This chart is a reference. To choose a conductor, first check ampacity in the wire ampacity chart, apply any derating, then confirm voltage drop. The wire size calculator does all of this in one step, and the conduit fill calculator picks the raceway.
Gauge vs ampacity
A conductor's ampacity is not fixed by its size alone. It depends on insulation temperature rating, the terminal rating, ambient temperature, the number of current-carrying conductors bundled together, and whether it is in free air or a raceway. The 60/75/90 °C figures above are for not more than three current-carrying conductors in raceway or cable at 30 °C ambient.