Wire Resistance & Length
Resistance of a wire from length, cross-section and material — copper, aluminium or your resistivity — corrected for temperature; or the length or area needed.
Resistance, Ω/km and Ω/1000 ft, the equivalent diameter, the resistivity and coefficient used, and with a current the voltage drop and power loss; length or area in the reverse solves.
Example: 100 m of 2.5 mm² copper at 20 °C: 0.690 Ω (6.90 Ω/km); at 10 A that drops 6.9 V and loses 69 W. A 1.5 mm² loop measuring 0.5 Ω is 21.8 m long (43.5 m of conductor).
ρ L over A,
warmed up.
The resistance law, the material constants and where they come from, and what changes with AC.
The law
A uniform conductor’s resistance is R = ρ·L/A: resistivity times length over cross-section. With ρ in Ω·mm²/m, L in metres and A in mm² the result is ohms directly. The reverse solves give the length that has a resistance (locating a fault, winding a coil) or the area that keeps a run under a limit.
Materials and temperature
Annealed copper is the International Annealed Copper Standard, 0.017241 Ω·mm²/m at 20 °C with α = 0.00393 per °C, from NBS Handbook 100 (a public-domain US government table); conductor-grade aluminium is 61 % IACS, 0.028264 Ω·mm²/m, α = 0.00403, as commonly published. Resistance rises linearly with temperature: R_T = R₂₀·(1 + α(T − 20)). Alloys, tinned or plated wire differ by a few percent — enter the data-sheet value as custom.
DC only
This is the DC resistance of a solid conductor. Stranding adds a lay factor of a percent or two; at higher frequencies skin and proximity effects raise the effective resistance; a round trip doubles the length. Voltage drop and power at a current are Ohm’s law on the result; ampacity is a separate question. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- R = ρ·L/A × (1 + α(T − 20)); copper 0.017241 Ω·mm²/m, α 0.00393 (NBS Handbook 100, public domain); aluminium 0.028264, α 0.00403 (61 % IACS as commonly published); DC resistance only
Last reviewed 21 September 2026. How results are checked: How we verify.