AWG 21 wire size, resistance and weight calculator
AWG 21 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
- 0.722 947
- Area in circular mils
- 0.810 114
- Resistance of that length
- 4.199 85 ohm
- Resistance per kilometre
- 41.9985 ohm/km
- Resistance per 1000 feet
- 12.8011 ohm
- Weight of that length
- 0.3678
- Current at the chosen density
- 1.64 A
- Turns per centimetre, close-wound
- 13.832
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 - (21)) / 39) = 0.722 947
- diameter_in = 0.722 947 / 25.4 = 0.028 462 5
- Cross-sectional area = pi * 0.722 947^2 / 4 = 0.410 491
- Area in circular mils = 0.028 462 5^2 * 1000 = 0.810 114
- r_per_m_20 = 1.724e-8 / (0.410 491 * 1e-6) = 0.041 998 5
- r_per_m = 0.041 998 5 * (1 + 0.00393 * (20 - 20)) = 0.041 998 5
- Resistance of that length = 0.041 998 5 * 100 = 4.199 85
- Resistance per kilometre = 0.041 998 5 * 1000 = 41.9985
- Resistance per 1000 feet = 0.041 998 5 * 304.8 = 12.8011
- Weight of that length = 0.410 491 * 1e-6 * 100 * 8960 = 0.3678
- Current at the chosen density = 0.410 491 * 4 = 1.641 96
- Turns per centimetre, close-wound = 0.722 947 > 0 ? 10 / 0.722 947 : 0 = 13.8323
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 21 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.