40 AWG wire size
0.08 mm · 0.005 mm²
0.0031 in diameter · 3440.87 Ω/km in copper
What 40 AWG is used for
The thinnest common magnet wire, used in miniature coils and sensors.
The numbers
| Diameter | 0.08 mm · 0.0031 in |
| Cross-sectional area | 0.005 mm² |
| Resistance, copper | 3440.87 Ω/km · 1048.777 Ω per 1,000 ft |
| Resistance, aluminium | 5628.337 Ω/km |
| NEC 60 °C copper ampacity | — outside the common building-wire range |
How 40 compares
The gauge scale is geometric, so the useful comparisons are ratios rather than differences. 34 AWG is twice the diameter, which is four times the area.
| Gauge | Diameter | Area | Ω/km copper | vs 40 |
|---|---|---|---|---|
| 34 | 0.16 mm | 0.02 mm² | 855.91 | 4.02× the area |
| 36 | 0.127 mm | 0.013 mm² | 1360.94 | 2.53× the area |
| 38 | 0.101 mm | 0.008 mm² | 2163.98 | 1.59× the area |
Frequently asked questions
What is 40 AWG in mm?
0.08 mm in diameter (0.0031 inches), with a cross-sectional area of 0.005 mm².
Is 40 AWG used for power wiring?
Not for building circuits. It is a signal and low-current gauge — thinner than anything a domestic branch circuit uses.
What is the resistance of 40 AWG copper?
3440.87 Ω per kilometre, or 1048.777 Ω per 1,000 feet, for annealed copper at 20 °C. The same gauge in aluminium is about 5628.34 Ω/km — 1.64 times higher, which is why aluminium wiring is sized larger for the same load.
Why do smaller AWG numbers mean thicker wire?
The number counts drawing operations. Wire is pulled through progressively smaller dies, and the gauge records how many passes it took — so more passes means a higher number and a thinner wire.