Wire Resistance Calculator

Yes, Wire Resistance Calculator is totally free :)

Yes, you can install the webapp as PWA.

Yes, any data related to Wire Resistance Calculator only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

Everything you type is normalised to SI before any arithmetic happens, and the whole page is then derived from one unrounded resistance. A gauge, diameter or radius is turned into a cross-sectional area, the length is doubled if you picked a round trip and divided by the number of strands and parallel runs, and R = ρL/A is evaluated once. Voltage drop, power loss, load voltage, conductance and the minimum-gauge recommendation all read that same figure, so nothing is ever rebuilt from a value you can see rounded on screen.

Because two different resistivities for copper at 20 °C are both in common use and they disagree by 2.6 %. The International Annealed Copper Standard value, 1.7241 × 10⁻⁸ Ω·m, is 100 % IACS by definition and is the copper behind every published AWG resistance table, cable datasheet and wiring code — the tool defaults to it, which is why 100 ft of 12 AWG comes out at 0.1588 Ω, the familiar 1.59 Ω per 1000 ft. High-purity laboratory copper is 1.68 × 10⁻⁸ Ω·m, the figure most physics textbooks quote; on the IACS scale it reads 102.6 % and gives resistances about 2.6 % under the wire charts. Pick IACS to check a real cable, pure copper to match a textbook answer.

Enter the physical run and let the round-trip toggle handle the rest. Current has to get to the load and back, so a 50 ft two-conductor cable is 100 ft of conductor as far as voltage drop is concerned, and forgetting the return leg halves every drop figure. Switch the run type to round trip and the tool doubles the length for you and shows the effective length it actually used, so the doubling is visible rather than assumed.

More than most people expect. Copper's coefficient is 0.00393 per °C, so a conductor running at 75 °C instead of the 20 °C its resistivity is quoted at carries about 21.6 % more resistance — a 0.242 Ω run becomes 0.294 Ω, and the voltage drop rises with it. The tool applies the linear model R_T = R₂₀·[1 + α(T − T₀)] and shows both figures side by side. That straight-line fit is excellent for metals from roughly −50 °C to 200 °C; outside that band it drifts, and the calculator blocks the result outright if the correction factor would fall to zero or below.

Aluminium's resistivity is 2.65 × 10⁻⁸ Ω·m against IACS copper's 1.7241 × 10⁻⁸, so it is only about 65 % as conductive and needs roughly 1.5 times the cross-section for the same resistance — in practice one to two AWG sizes larger. What it gives back is weight: at 2700 kg/m³ against copper's 8960 it is under a third of the mass, so per kilogram of metal aluminium wins outright. That is why utility distribution and large service entrances use it while branch wiring generally does not.

For a solid round conductor at DC it is exact — R = ρL/A is not an approximation. Real stranded cable typically reads 2–3 % higher than the geometric figure, because a stranded conductor's individual wires spiral and so are longer than the cable, and because the nominal area is rarely the exact area. At mains frequency skin effect is negligible below about 4 AWG but becomes real in large conductors, and the tool models DC only. Contact and termination resistance is not included at all, and on short low-voltage runs that can rival the wire itself.