Wire Gauge Calculator

American Wire Gauge numbers run backward: smaller numbers mean thicker wire. This tool does two jobs. In sizing mode, enter the load current and, optionally, the run length and voltage, and it finds the smallest copper or aluminum wire that meets the National Electrical Code ampacity table and keeps voltage drop under your limit. In lookup mode, pick a gauge to see its diameter, cross-sectional area in mm² and circular mils, resistance per 1,000 feet and per kilometer, ampacity and the largest breaker it may be protected by.

The chart shows how ampacity climbs across standard building wire sizes from 14 AWG to 4/0.

Diameter (in) = 0.005 × 92^((36 − n) ÷ 39); 1/0 is n = 0, 4/0 is n = −3

Circular mils = (diameter in mils)²; resistance = K × 1,000 ÷ circular mils per 1,000 ft

The AWG formula, standardized in ASTM B258, makes each step about 26% larger in area; every six gauges roughly doubles the diameter and every three doubles the area. Resistance uses K = 12.9 Ω·cmil/ft for copper and 21.2 for aluminum at 75 °C. Ampacity comes from the 75 °C column of NEC Table 310.16, which applies when terminations are rated 75 °C, and the 15, 20 and 30 A limits for 14, 12 and 10 AWG copper come from NEC 240.4(D).

A water heater. 30 A at 240 V over 50 ft needs 10 AWG copper; the run is short enough that ampacity governs.

A long 20 A circuit. 16 A over 150 ft at 120 V: ampacity alone allows 12 AWG, but keeping drop under 3% requires 6 AWG.

An aluminum feeder. 100 A over 80 ft at 240 V needs 1 AWG aluminum.

Table ampacities assume no more than three current-carrying conductors in a raceway or cable and an ambient temperature of 30 °C. More conductors together, hot attics or rooftops, and insulation that traps heat all reduce the allowable current through the NEC adjustment and correction factors. Continuous loads, those running three hours or more such as EV chargers and water heaters, must be sized at 125% of the load. Many residential cables such as NM-B are limited to the 60 °C column. Always check equipment instructions and your local code, and have an electrician verify any installation.

Building-wire ampacity tables do not apply to thin hookup wire, speaker cable or automotive wiring, where insulation types, bundling and short runs differ. For those, voltage drop and heating in free air usually decide the size. Speaker runs are typically 16 AWG for short distances and 12 or 14 AWG for long runs to low-impedance speakers. Automotive and marine circuits are sized with tables that combine current and round-trip length, aiming for 3% drop on critical circuits and 10% on others. For 12 V and 24 V solar and battery systems, the voltage drop calculator is usually the better guide, since even modest currents lose a large share of a low voltage.

Outside North America, wire is sold by cross-sectional area in square millimeters, in steps such as 1.5, 2.5, 4, 6 and 10 mm². The nearest matches are roughly 16, 14, 12, 10 and 8 AWG, but they are not identical, so compare areas from the lookup mode rather than assuming a direct swap. Metric installations follow IEC 60364 and national wiring rules with their own current ratings, which differ from the NEC values used here.