Wire Resistance Calculator

How it is calculated

The tool uses R = ρ × L / A. Length L is in metres and the cross-section area A in mm², which the tool converts to m² (× 10⁻⁶). The resistivity ρ at room temperature comes from the material you pick, in Ω·m: copper 1.68e-8, aluminium 2.65e-8, gold 2.44e-8, silver 1.59e-8. An unknown material is treated as copper. The result is in ohms with 4 significant digits. Area must be above zero, and length cannot be negative. Resistance rises with temperature and the tool does not correct for it. For a two-wire circuit, enter the full out-and-back length.

Common mistakes

• Entering the area in m² or as the wire diameter. The field is the cross-section area in mm², so a 2.5 mm² cable is 2.5 and a diameter must be turned into area with π × d² / 4 first.
• Entering one-way length for a circuit that has go and return wires. Resistance is of the entire length, so double the distance.
• Choosing copper for an aluminium cable. Aluminium resists about 58% more for the same size.
• Ignoring heat. Warm wires have higher resistance than the room-temperature value.