Last updated October 10, 2026 · Report an error
Every wire has some resistance, determined by what it's made of, how long it is, and how thick it is. This calculator finds a copper or aluminum wire's resistance from its length and cross-sectional area.
How it works
Choose copper or aluminum, enter the wire's length and cross-sectional area, and the calculator applies R = ρL/A, using each material's standard resistivity at 20°C.
- Enter wire material.
- Enter wire length (m).
- Enter cross-sectional area (mm²).
- Click Calculate to see your results.
Examples
A thin copper wire
A 10 m length of copper wire with a 1 mm² cross-section has a resistance of about 0.168 Ω — noticeable in long, thin wiring runs, but usually negligible for short, thick ones.
Who should use it
- Electrical wiring and circuit design coursework.
- Estimating resistance losses in long wiring runs.
Industry applications
- Electrical wiring and installation
- Electronics and circuit design
Advantages
- Covers the two most common wiring materials with standard, correct resistivity values.
- Simple, direct calculation from length and cross-sectional area.
Limitations
- Doesn't account for temperature effects on resistance, or skin effect at very high frequencies.
Common mistakes to avoid
- Using diameter or radius directly as the cross-sectional area — area is calculated from those (πr²), not the same number.
- Ignoring wire resistance in long runs where voltage drop actually matters (see the Voltage Drop Calculator for that specific check).
Best practices
- For long wire runs carrying meaningful current, check the resulting voltage drop, not just the raw resistance value, to see if it actually matters for your application.
- Remember this uses resistivity at 20°C — real-world resistance rises somewhat at higher operating temperatures.
Tips
- Pair this with the Voltage Drop Calculator to see the actual practical impact of a wire's resistance at your specific operating current.
Frequently asked questions
Yes, with no signup and no limit on how many calculations you run.
Copper has a lower resistivity (about 1.68×10⁻⁸ Ω·m versus aluminum's 2.65×10⁻⁸ Ω·m) — its atomic structure lets electrons flow with less scattering/resistance, which is why copper is preferred for most wiring despite costing more than aluminum.
A thicker wire gives electrons more "room" to flow, spreading the current over a larger area and reducing resistance — resistance is inversely proportional to cross-sectional area.
Yes — this calculator uses standard resistivity values at 20°C; resistance rises somewhat as a wire heats up, which matters for high-current or high-ambient-temperature applications, but isn't accounted for in this simplified calculation.