AWG Wire Gauge Calculator

What Is AWG?

AWG stands for American Wire Gauge, also called Brown & Sharpe gauge, the standard wire sizing system used in North America since 1857. It runs backwards from what you would expect: a higher AWG number means a thinner wire. AWG 40 is about the thickness of a human hair, while AWG 0000, spoken as four-ought, is 11.7 mm across and carries hundreds of amps.

The Mathematical Formula Behind AWG

AWG is based on a logarithmic scale. The formula for diameter in millimeters is:

diameter (mm) = 0.127 × 92^((36 − AWG) / 39)

Every 6 AWG numbers, the wire diameter doubles. Every 3 AWG numbers, the wire cross-sectional area (and therefore current capacity) doubles. This geometric progression means that jumping from AWG 14 to AWG 8 gives you four times the cross-sectional area.

Why Does Wire Gauge Matter for Safety?

Resistance is the key. A thinner wire has higher resistance per unit length, and current flowing through resistance makes heat (P = I²R). Push too much current through too thin a wire and the insulation melts, which is how electrical fires start. That is why a circuit needs both correctly sized wire and a correctly sized breaker: the breaker has to trip before the wire overheats, so the wire must be rated above the breaker, never below it.

NEC (National Electrical Code) Ampacity

Table 310.16 gives ampacity in columns by insulation temperature rating, and the column you use is decided by the terminations, not by the wire. Under NEC 110.14(C), a circuit is sized to the lowest-rated component in it, and most breakers and devices rated 100 A or less are listed at 60°C. That is why the conservative column is the one that usually governs household work.

The familiar household pairings come from the 60°C column: AWG 14 on a 15 amp circuit, AWG 12 on 20 amps for kitchen and bathroom receptacles, AWG 10 on 30 amps for a dryer or water heater, AWG 8 on 40 amps for a range.

Two things these numbers assume: no more than three current-carrying conductors in the raceway, and a 30°C ambient. Bundle more conductors or run through a hot roof space and the figures come down, sometimes sharply.

Copper vs. Aluminum

Copper has lower resistivity (1.72×10⁻⁸ Ω·m) than aluminum (2.65×10⁻⁸ Ω·m), meaning copper wire can carry more current at the same gauge. Aluminum wire is cheaper and lighter, but requires larger gauge for the same ampacity, and special connectors to prevent oxidation at junctions. Most residential wiring uses copper. Large service entrance cables are often aluminum to reduce cost and weight.

Metric equivalent (mm²)

Most of the world sells wire by cross-sectional area in mm², and AWG sizes do not land on the standard metric ones. The calculator above gives the true area; the table below gives the metric cable you would actually buy, which is the next size up:

So if the calculator says AWG 12 is 3.31 mm² and a supplier offers you 4 mm², that is the right cable and not an error. Substituting downward, 2.5 mm² for AWG 12, would leave you under-sized.