American Wire Gauge runs backwards: the larger the number, the thinner the wire. The scale is geometric, defined so that #36 measures 0.005 inches and #0000 measures 0.46 inches, with 39 equal ratio steps between them.
Two consequences are worth memorising. Six gauge numbers doubles the diameter, which quadruples the cross-sectional area. Three gauge numbers doubles the area. So 14 AWG has twice the copper of 17 AWG, and four times that of 20 AWG.
Why area is the number that matters
Resistance follows R = ρL/A. Copper’s resistivity ρ is 1.68 × 10⁻⁸ Ω·m at 20 °C, L is the length and A the cross-sectional area. Because area scales with the square of the diameter, a small change in gauge produces a large change in resistance — and in the heat the wire generates, since that goes as I²R.
Two different limits
Wire sizing is constrained by two separate things, and which one binds depends on the run.
- Heating. The wire must not get hot enough to damage its insulation. This dominates on short runs at high current.
- Voltage drop. The wire must not eat so much of the supply voltage that the load misbehaves. This dominates on long runs, and it is why a wire that is thermally fine can still be the wrong choice — a 12 V circuit losing 1 V in the cable has lost over 8% of its supply.
Remember to count both conductors. Current flows out along one and back along the other, so a 10 metre run means 20 metres of copper for voltage-drop purposes.
Common sizes
| AWG | Diameter | Area | Ω per metre | Typical use |
|---|---|---|---|---|
| 10 | 2.59 mm | 5.26 mm² | 0.0032 | High-current DC, battery leads |
| 12 | 2.05 mm | 3.31 mm² | 0.0051 | Mains circuits, motors |
| 14 | 1.63 mm | 2.08 mm² | 0.0081 | Lighting circuits |
| 18 | 1.02 mm | 0.82 mm² | 0.0205 | Hookup wire, low-power loads |
| 22 | 0.64 mm | 0.33 mm² | 0.0518 | Breadboards, signal wiring |
| 24 | 0.51 mm | 0.20 mm² | 0.0842 | Jumper leads, ribbon cable |
| 30 | 0.25 mm | 0.051 mm² | 0.339 | Wire-wrap, fine work |
What this calculator does not account for
Copper resistance rises about 0.39% per degree Celsius, so a wire running at 70 °C has roughly 20% more resistance than the 20 °C figure used here. Stranded wire has slightly more resistance than solid of the same nominal gauge, because the strands spiral and are therefore longer than the cable. At high frequencies the skin effect pushes current into the outer surface, making the effective area smaller than the physical one — irrelevant at DC and mains frequency, significant in RF work.
Metric cable is specified directly by cross-sectional area in mm² rather than by gauge number, which sidesteps the whole conversion. 2.5 mm² sits between 13 and 14 AWG; 1.5 mm² is close to 15 AWG.