Wire Resistance Calculator (Copper and Aluminium)
By Santaji GadepatilinUpdated October 28, 2026
Every conductor has resistance that rises with length and temperature and falls with cross-section. It sets voltage drop, cable heating and the burden on instrument loops. This wire resistance calculator handles copper and aluminium at any temperature — see what resistance is and electrical conductivity explained.
How to use the wire resistance calculator
Choose the material, enter the total conductor length, the cross-section and the operating temperature. You get the resistance at 20 °C, at your temperature, and per kilometre.
Wire resistance formula
R = ρ × L ÷ A · RT = R₂₀ × [1 + α(T − 20)]
The calculator uses α = 0.00393 /°C, the temperature coefficient of copper; aluminium (0.00403 /°C) is very close.
Copper conductor resistance at 20 °C
| Size | Ω/km (calculated) | Typical IEC 60228 max (Class 2) |
|---|---|---|
| 0.5 mm² | 34.48 | 36.0 |
| 0.75 mm² | 22.99 | 24.5 |
| 1 mm² | 17.24 | 18.1 |
| 1.5 mm² | 11.49 | 12.1 |
| 2.5 mm² | 6.90 | 7.41 |
| 4 mm² | 4.31 | 4.61 |
| 6 mm² | 2.87 | 3.08 |
| 10 mm² | 1.72 | 1.83 |
| 16 mm² | 1.08 | 1.15 |
| 25 mm² | 0.69 | 0.727 |
Real stranded conductors are slightly higher than the solid-wire calculation because of stranding and compaction.
AWG equivalents
| AWG | Area | Copper Ω/km |
|---|---|---|
| 24 | 0.205 mm² | 84.2 |
| 20 | 0.519 mm² | 33.3 |
| 18 | 0.823 mm² | 21.0 |
| 16 | 1.31 mm² | 13.2 |
| 14 | 2.08 mm² | 8.29 |
| 12 | 3.31 mm² | 5.21 |
| 10 | 5.26 mm² | 3.28 |
Uses
- Voltage drop — see the cable voltage drop calculator.
- 4–20 mA loop budgets — see the loop resistance calculator.
- RTD lead-wire error: 2-wire RTDs read high by the lead resistance (about 0.39 °C per 0.15 Ω for a Pt100).
Frequently asked questions
How do I calculate wire resistance?
R = ρ × L ÷ A, where ρ is the resistivity, L the length and A the cross-section. 100 m of 1.5 mm² copper has about 1.15 Ω at 20 °C.
How does temperature affect resistance?
Copper and aluminium resistance rises about 0.39–0.40% per °C. At 70 °C, copper resistance is about 20% higher than at 20 °C.
Should I use the one-way or round-trip length?
For a circuit, use the total conductor length — twice the one-way distance for a two-wire circuit.
Why is aluminium cable larger than copper?
Aluminium's resistivity is about 64% higher, so it needs roughly 1.6 times the cross-section for the same resistance.