AWG 18 wire size, resistance and weight calculator
AWG 18 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
- 1.023 69
- Area in circular mils
- 1.6243
- Resistance of that length
- 2.094 66 ohm
- Resistance per kilometre
- 20.9466 ohm/km
- Resistance per 1000 feet
- 6.384 51 ohm
- Weight of that length
- 0.737 45
- Current at the chosen density
- 3.29 A
- Turns per centimetre, close-wound
- 9.769
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 - (18)) / 39) = 1.023 69
- diameter_in = 1.023 69 / 25.4 = 0.040 302 7
- Cross-sectional area = pi * 1.023 69^2 / 4 = 0.823 047
- Area in circular mils = 0.040 302 7^2 * 1000 = 1.6243
- r_per_m_20 = 1.724e-8 / (0.823 047 * 1e-6) = 0.020 946 6
- r_per_m = 0.020 946 6 * (1 + 0.00393 * (20 - 20)) = 0.020 946 6
- Resistance of that length = 0.020 946 6 * 100 = 2.094 66
- Resistance per kilometre = 0.020 946 6 * 1000 = 20.9466
- Resistance per 1000 feet = 0.020 946 6 * 304.8 = 6.384 51
- Weight of that length = 0.823 047 * 1e-6 * 100 * 8960 = 0.737 45
- Current at the chosen density = 0.823 047 * 4 = 3.292 19
- Turns per centimetre, close-wound = 1.023 69 > 0 ? 10 / 1.023 69 : 0 = 9.768 61
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
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 18 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.