About the Wire Resistance Calculator
This wire resistance calculator works out the DC resistance of a length of wire from its material, size and length. Enter the size as an AWG gauge, a cross-sectional area in mm² or a bare diameter, choose feet or metres, and it returns the total resistance along with ohms per 1,000 ft and per km, the conductor area in mm² and circular mils, and — if you enter a current — the voltage drop and the heat dissipated in the wire.
It is useful for sizing power leads for LED strips, solar panels, battery banks and car audio, estimating losses in long sensor or speaker runs, and checking whether a thin hookup wire is good enough for a motor or heater. Tick the round-trip option to count both the outgoing and return conductors, which is what matters for voltage drop in a two-wire circuit.
Resistivities are standard 20 °C values (annealed copper per the International Annealed Copper Standard, 1.7241 × 10⁻⁸ Ω·m; EC-grade aluminum at 61% IACS). A linear temperature coefficient adjusts the result for hotter or colder wire. Results are for solid or stranded wire of the stated cross-section at DC; skin effect raises resistance at radio frequencies.
With the default inputs, the total wire resistance is 0.1588 Ω. Change any value above to recalculate instantly.
How to use the wire resistance calculator
- 1Choose the conductor material — copper for most hookup and building wire.
- 2Pick how to enter the size: AWG gauge, area in mm² or diameter in mm.
- 3Enter the length and unit, and tick round trip if the length is the one-way distance of a two-wire run.
- 4Set the wire temperature if it runs hot or cold.
- 5Enter the load current to see voltage drop and power loss, and compare nearby gauges in the table.
Formula and method
The resistance of a uniform conductor is its resistivity times its length divided by its cross-sectional area. The calculator converts the size you give into an area: AWG numbers use the defining geometric formula in which diameter grows by a factor of 92 over the 39 steps from AWG 36 (0.005 in) to AWG 4/0 (0.46 in), and a diameter d gives A = πd²/4.
Resistivity rises almost linearly with temperature, so ρ at 20 °C is scaled by 1 + α(T − 20). Copper (α ≈ 0.00393/°C) is about 23% more resistive at 80 °C than at 20 °C. Voltage drop is Ohm’s law V = I × R and the heat in the wire is P = I² × R. Ticking “round trip” doubles the length so that both supply and return conductors are counted.
- R
- Total resistance (Ω)
- ρ₂₀
- Resistivity at 20 °C (Ω·m); copper 1.7241 × 10⁻⁸, aluminum 2.8264 × 10⁻⁸
- α
- Temperature coefficient of resistance (per °C)
- L
- Conductor length in metres (doubled for round trip)
- A
- Cross-sectional area in m²
- d
- Bare conductor diameter
Worked examples
100 ft of 12 AWG copper at 10 A
12 AWG has a diameter of 2.0525 mm, so A = 3.309 mm². R per metre = 1.7241 × 10⁻⁸ ÷ 3.309 × 10⁻⁶ = 5.21 mΩ, or 1.588 Ω per 1,000 ft. 100 ft (30.48 m) is therefore 0.159 Ω, and 10 A drops 1.59 V while heating the wire by 15.9 W.
20 m round trip of 2.5 mm² copper at 60 °C, 16 A
At 20 °C 2.5 mm² copper is 6.896 Ω/km; at 60 °C it rises by a factor of 1 + 0.00393 × 40 = 1.157 to 7.98 Ω/km. A 20 m cable has 40 m of conductor, giving 0.319 Ω — 16 A drops 5.11 V (2.2% of 230 V) and wastes about 82 W.
50 ft of 4 AWG aluminum battery cable
4 AWG is 5.189 mm in diameter (21.15 mm²). Aluminum at 2.8264 × 10⁻⁸ Ω·m gives 1.336 Ω/km or 0.407 Ω per 1,000 ft, so 50 ft is 0.0204 Ω and 100 A loses 2.04 V.
Frequently asked questions
What is the resistance of 12 gauge copper wire?+
Solid or stranded 12 AWG copper is about 1.59 Ω per 1,000 ft (5.21 Ω per km) at 20 °C. A 100 ft length is roughly 0.16 Ω, or 0.32 Ω if you count both conductors of a 100 ft circuit.
How does temperature change wire resistance?+
Metal resistance rises roughly linearly with temperature. Copper increases about 0.39% per °C, so wire at 75 °C has about 22% more resistance than at 20 °C. Building-wire ampacity tables account for this by assuming hot conductors.
Is aluminum wire more resistive than copper?+
Yes. Aluminum conducts about 61% as well as copper, so for the same resistance you need roughly 1.6 times the cross-section — typically two AWG sizes larger. It is lighter and cheaper per ampere, which is why it is used for service entrances and overhead lines.
Why does AWG get thicker as the number gets smaller?+
AWG numbers came from the number of drawing steps used to make the wire: thinner wire needed more passes through dies. Each step down of three gauges roughly doubles the area and halves the resistance; six gauges halves the resistance twice over to a quarter.
Do I need to double the length for voltage drop?+
For a two-wire DC or single-phase circuit, current flows out on one conductor and back on the other, so the total resistance is twice the one-way length. Tick the round-trip option when you enter the one-way distance.
Does stranded wire have higher resistance than solid wire?+
Slightly. Stranded wire of the same gauge has a similar copper area, but the lay of the strands makes each strand a little longer, adding about 2–3% resistance. For most calculations the difference is negligible.