AWG 40 wire size, resistance and weight calculator
AWG 40 is defined by the gauge formula, not by a table: the diameter is 0.127 mm times 92 raised to the power (36 - n) / 39. Everything else follows from that and the conductor resistivity.
A rough guide: 3 to 5 A/mm2 for chassis wiring, under 2 for long runs in conduit.
- Diameter
- 0.079 871 1
- Area in circular mils
- 0.009 888 07
- Resistance of that length
- 344.087 ohm
- Resistance per kilometre
- 3440.87 ohm/km
- Resistance per 1000 feet
- 1048.78 ohm
- Weight of that length
- 0.004 489 28
- Current at the chosen density
- 0.02 A
- Turns per centimetre, close-wound
- 125.202
Show the working
- Length of wire = 100
- Conductor = 1.724e-8
- Conductor temperature = 20
- Conductor density = 8960
- Design current density = 4
- Diameter = 0.127 * 92^((36 - (40)) / 39) = 0.079 871 1
- diameter_in = 0.079 871 1 / 25.4 = 0.003 144 53
- Cross-sectional area = pi * 0.079 871 1^2 / 4 = 0.005 010 36
- Area in circular mils = 0.003 144 53^2 * 1000 = 0.009 888 07
- r_per_m_20 = 1.724e-8 / (0.005 010 36 * 1e-6) = 3.440 87
- r_per_m = 3.440 87 * (1 + 0.00393 * (20 - 20)) = 3.440 87
- Resistance of that length = 3.440 87 * 100 = 344.087
- Resistance per kilometre = 3.440 87 * 1000 = 3440.87
- Resistance per 1000 feet = 3.440 87 * 304.8 = 1048.78
- Weight of that length = 0.005 010 36 * 1e-6 * 100 * 8960 = 0.004 489 28
- Current at the chosen density = 0.005 010 36 * 4 = 0.020 041 4
- Turns per centimetre, close-wound = 0.079 871 1 > 0 ? 10 / 0.079 871 1 : 0 = 125.202
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- diameter in mm = 0.127 x 92^((36 - n)/39)
- area = pi d^2 / 4
- resistance = rho x length / area
- temperature correction: R(T) = R(20) x (1 + 0.00393 (T - 20)) for copper
The gauge formula is exact: 36 AWG is 0.127 mm and 0000 AWG is 11.684 mm, with 39 steps between them in a geometric progression of ratio 92^(1/39).
The current figure is a design rule of thumb from current density, not an ampacity rating. Real ampacity depends on insulation temperature rating, bundling, ambient temperature and the wiring code in force.
Aluminium conductors of the same gauge carry about 61% of the current of copper for the same heating.
Frequently asked
What is the diameter of AWG 40 wire?
It follows from the gauge definition: 0.127 mm x 92^((36 - n)/39). The calculator shows the result in millimetres and inches.
Why does the wire gauge number go down as the wire gets thicker?
The number counts drawing operations. Each pass through a die makes the wire thinner and adds one to the gauge, so a high number means a thin wire.