Why does a bigger AWG number mean a thinner wire?

The number originally counted drawing operations: each pass through a die made the wire thinner, so more passes meant a higher number and a smaller diameter.

What voltage drop is acceptable?

Commonly three percent for a final circuit and five percent overall, though the applicable regulation governs. Long runs fail on drop well before they fail on current.

ConversionstechnicalUnited States only

Wire Gauge Converter

Wire gauge to diameter, with the voltage drop over a run. AWG is a geometric series running backwards, so a larger number is a thinner wire and every six gauges halves the diameter.

Also called: awg to mm converter, wire size calculator.

Diameter

2.05

AWG 12 is 2.05 mm across, a cross section of 3.31 mm². Over a 15 m run at 20 A the voltage drop is 3.05 V, which is 1.33% of 230 V. That is within the three percent commonly allowed for a final circuit.

AWG gauge

Cross section

3.31

Resistance per km

5.08

Voltage drop

3.05

Drop as a percentage

1.33

On the drop

That is within the three percent commonly allowed for a final circuit.

On the gauge system

AWG runs backwards: six gauges halves the diameter and three gauges halves the cross section.

How this is calculated

Method and background

How this is calculated

AWG is defined by a formula rather than a table: gauge 36 is 0.005 inches and gauge 0000 is 0.46, with 39 steps between them in geometric progression. That gives the two rules of thumb worth remembering: six gauges halves the diameter, and three gauges halves the cross-sectional area. Voltage drop matters more than current rating on long runs, and the calculation uses twice the one-way length because the current returns.

AWG is a geometric series, which is why the numbers run backwards and why six gauges halve the diameter

n: AWG number

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

AWG 12

Gauge or diameter: 12
That is: AWG gauge
Current to carry: 20
Run length one way: 15
System voltage: 230

Diameter2.05

0.127 x 92^(24/39)

Open this example

six gauges halves the diameter

Gauge or diameter: 18
That is: AWG gauge
Current to carry: 20
Run length one way: 15
System voltage: 230

Diameter1.02

boundary: 2.0525 / 2 to within rounding, the defining property of the series

Open this example

Method and limits

What it assumes

Copper conductor at 20 degrees Celsius, and a DC or single-phase resistive load.

What it deliberately does not model

Current-carrying capacity depends on insulation, bundling and ambient temperature, not on the gauge alone.
Aluminium conductors have about 1.6 times the resistance of copper for the same size.
This is a conversion, not a substitute for the applicable wiring regulations.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why does a bigger AWG number mean a thinner wire?: The number originally counted drawing operations: each pass through a die made the wire thinner, so more passes meant a higher number and a smaller diameter.
What voltage drop is acceptable?: Commonly three percent for a final circuit and five percent overall, though the applicable regulation governs. Long runs fail on drop well before they fail on current.