DC Wire Size Calculator
Use our DC Wire Size Calculator to know the wire size needed for your DC electric application.
The formula
R = ρL ÷ A; AWG diameter = 0.127 mm × 92^((36−n)/39)
The gauge scale runs backwards, and geometrically
American Wire Gauge numbers get smaller as the wire gets thicker, and the series is geometric: 36 AWG is 0.005 inches, and each 39 gauges spans a factor of 92 in diameter. Two useful consequences fall out of that. Three gauges down doubles the cross-sectional area, and six gauges down quadruples it — so six gauges down quarters the resistance, almost exactly.
Because the formula is exact, this page computes diameters rather than looking them up. Published tables round, and rounded diameters get squared into areas, which compounds the error.
Voltage drop is usually what sizes a cable, not the current rating. A long run at modest current can easily exceed the customary 3% limit while remaining far below the wire's thermal capacity, and the fix is a thicker conductor. Remember the current goes out and comes back, so the resistance that matters is the round trip — using the one-way length halves the answer.
Temperature matters more than people expect. Copper's resistivity rises about 0.4% per degree, so a cable running at 75 °C has roughly 22% more resistance than the 20 °C table value. Sizing from cold figures understates the drop under load.