AWG Wire Gauge
Diameter, area, resistance, and voltage drop for any AWG size.
20.95 Ω per km of copper at 20 °C. Each 6 gauges roughly halves the diameter and quarters the area.
209 mΩ one way, doubled for the return path, at 2 A. Power lost as heat: 1.68 W.
Comfortable for both short chassis wiring and longer power runs.
The current limits shown are the widely quoted hobby figures, not code ampacity. A real installation derates for insulation temperature, how many conductors run in a bundle, and ambient heat — always check the applicable electrical code for mains wiring.
American Wire Gauge runs backwards: a bigger number means thinner wire. AWG 12 is 2.053 mm across with 5.21 Ω per kilometre, while AWG 24 is 0.511 mm and 84.2 Ω per kilometre — sixteen times the resistance.
A geometric scale, not an arbitrary list
AWG numbers look like a lookup table but are defined by a formula. The standard fixes AWG 36 at 0.005 inches and AWG 0000 at 0.46 inches, with 39 equal geometric steps between them. Every gauge is therefore the previous one multiplied by 92^(1/39) ≈ 1.1229.
Two consequences fall out and are worth memorising. Six gauges halves the diameter — AWG 18 is half the diameter of AWG 12. And since area goes as diameter squared, three gauges halves the area, which means three gauges also doubles the resistance.
Why the numbering is backwards
The scale comes from wire drawing: wire is made by pulling it through progressively smaller dies, and the gauge number counted how many draws it had been through. More draws meant thinner wire, so a higher number is a thinner conductor — a manufacturing artefact preserved for over a century.
ρ for annealed copper is 1.724 × 10⁻⁸ Ω·m at 20 °C. The drop counts twice the run length because current flows out and back.
Worked example: 10 m of AWG 18 at 2 A
Diameter, then area, then the drop over the round trip:
- 1 Find the diameter. 0.127 × 92^((36 − 18) ÷ 39) = 1.024 mm.
- 2 Work out the area. π × (1.024 ÷ 2)² = 0.823 mm².
- 3 Get the resistance per kilometre. 1.724e−8 Ω·m ÷ 0.823 mm² gives about 20.9 Ω per km.
- 4 Scale to the run length. 20.9 Ω/km × 10 m = 0.209 Ω one way.
- 5 Double it for the return path. Current has to come back, so the loop is 0.418 Ω.
- 6 Apply Ohm’s law. 0.418 Ω × 2 A = 0.84 V dropped in the cable, wasting about 1.7 W as heat.
Common gauges
Copper at 20 °C. The current figures are the widely quoted hobby rules of thumb, not code ampacity.
| AWG | Diameter | Area | Ω / km | Chassis · power |
|---|---|---|---|---|
| 10 | 2.588 mm | 5.26 mm² | 3.28 | 55 A · 15 A |
| 12 | 2.053 mm | 3.31 mm² | 5.21 | 41 A · 9.3 A |
| 14 | 1.628 mm | 2.08 mm² | 8.28 | 32 A · 5.9 A |
| 18 | 1.024 mm | 0.823 mm² | 20.9 | 16 A · 2.3 A |
| 22 | 0.644 mm | 0.326 mm² | 52.9 | 7 A · 0.92 A |
| 24 | 0.511 mm | 0.205 mm² | 84.2 | 3.5 A · 0.58 A |
Two current limits, and why they differ so much
The table gives two figures because the constraint depends on the situation. The chassis number applies to a short run in open air where heat escapes easily. The power transmission number is far lower and applies to long runs, bundles, and conduit, where heat accumulates and voltage drop over distance starts to matter more than the wire's own temperature.
Neither is a code ampacity. Real installations derate for the insulation's temperature rating, how many conductors share a bundle or conduit, and ambient temperature — a cable rated comfortably on its own can be unsafe in a bundle of twelve. For mains wiring, the applicable electrical code is the authority, not a rule of thumb.
For low-voltage work the binding limit is usually voltage drop rather than heat. Losing 0.84 V is trivial on a 230 V supply but is 7% of a 12 V system, which will dim lights and make motors sluggish. A common target is keeping the drop under 3%, and that frequently demands a thicker wire than temperature alone would require.