Find the resistance of a wire from its material, gauge or diameter, length and temperature. Add a current to see the voltage drop and heat loss, and compare neighbouring AWG sizes.
Last updated:
× 10⁻⁸ Ω·m
per °C
Wire size given as
mm
in
mm²
For a circuit run: out and back
Optional: voltage drop
A
V
Enter your values to see the result.
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How to use the Wire Resistance Calculator
Pick the material (or enter a custom resistivity and α).
Choose how you give the size: AWG, diameter or area.
Enter the length; tick round trip for a two-wire circuit.
Set the temperature the wire runs at.
Optionally add current and voltage to see the drop and heat loss.
AWG diameter: d = 0.127 mm × 92(36 − n)/39, A = π d² / 4
Voltage drop and heat: V = I × R, P = I² × R
Example: 100 ft (30.48 m) of 12 AWG copper at 20 °C. A = 3.309 mm², so R = 1.724 × 10⁻⁸ × 30.48 / 3.309 × 10⁻⁶ ≈ 0.159 Ω. At 15 A that drops 2.4 V. Counted as a round trip (two 100 ft conductors), the drop doubles to 4.8 V, 4% of 120 V.
Resistivity of conductor materials (20 °C)
Material
Resistivity ρ (Ω·m)
α (per °C)
Silver
1.59 × 10⁻⁸
0.0038
Copper, annealed
1.724 × 10⁻⁸
0.00393
Gold
2.44 × 10⁻⁸
0.0034
Aluminum (1350 conductor)
2.826 × 10⁻⁸
0.00403
Tungsten
5.6 × 10⁻⁸
0.0045
Nickel
6.99 × 10⁻⁸
0.006
Brass (typical)
about 7 × 10⁻⁸
0.0015
Iron
9.71 × 10⁻⁸
0.0065
Constantan
4.9 × 10⁻⁷
about 0.00001
Stainless steel 304
7.2 × 10⁻⁷
0.00094
Nichrome 80/20
1.10 × 10⁻⁶
0.0004
Alloy values vary with exact composition and temper. The linear α model is accurate within a few percent over roughly −50 °C to 150 °C for common conductors.
Copper resistance by AWG (20 °C)
AWG
Diameter
Area
Ω per 1,000 ft
Ω per km
4
5.189 mm
21.15 mm²
0.248
0.815
6
4.115 mm
13.30 mm²
0.395
1.296
8
3.264 mm
8.37 mm²
0.628
2.061
10
2.588 mm
5.26 mm²
0.999
3.277
12
2.053 mm
3.31 mm²
1.588
5.210
14
1.628 mm
2.08 mm²
2.525
8.285
18
1.024 mm
0.823 mm²
6.385
20.95
22
0.644 mm
0.326 mm²
16.14
52.96
These are solid-wire figures. Stranded wire of the same gauge has very slightly higher resistance, and a wire carrying current runs warmer than the room, so real-world resistance is usually a little higher.
Frequently asked questions
How do you calculate the resistance of a wire?
Use R = ρ × L / A, where ρ is the material's resistivity in Ω·m, L the length in metres and A the cross-section area in m². Then correct for temperature with R_T = R₂₀ × (1 + α(T − 20 °C)).
What is the resistance of 12 AWG copper wire?
12 AWG copper (2.053 mm, 3.31 mm²) is about 1.59 Ω per 1,000 ft (5.21 Ω per km) at 20 °C. 100 ft of it is about 0.16 Ω.
How much does temperature change wire resistance?
Copper's resistance rises about 0.393% per °C. At 75 °C copper wire has about 22% more resistance than at 20 °C; at −20 °C it has about 16% less.
Is aluminum wire resistance higher than copper?
Yes. Aluminum conductor has about 1.64 times the resistivity of annealed copper (roughly 61% IACS conductivity), so an aluminum wire needs about two AWG sizes larger to match copper's resistance.
Why tick the round-trip option?
Current flows out on one conductor and back on the other, so the circuit resistance includes both. For a 50 ft run to a load, the wire resistance is that of 100 ft.
How is AWG converted to diameter?
d = 0.127 mm × 92^((36 − n) / 39). Every 3 gauges smaller roughly halves the area and doubles the resistance; every 6 gauges halves the diameter.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.