Wire Resistance Calculator
Resistance of a wire from length, diameter or AWG gauge, material and temperature — ohms total and per metre, voltage drop and power loss at your current.
How it works
R = ρ·L ÷ A, A = π(d/2)²; ρ(T) = ρ₂₀[1 + α(T − 20)]; V_drop = I·R; P = I²R
Resistance grows with length and shrinks with cross-section; doubling the diameter quarters the resistance. Copper’s resistivity rises about 0.4% per degree, so a hot cable drops noticeably more voltage than a cold one. For circuit design the useful outputs are the voltage drop at working current — keep it under about 3% of supply — and the heat the run dissipates, which is what cable ampacity tables protect against.
Worked example
25 m of AWG 14 copper (1.628 mm) as a two-wire run = 50 m: 0.404 Ω at 20 °C. At 15 A that drops 6.1 V (2.6% of 230 V) and dissipates 91 W along the cable.
Frequently asked questions
Why count the return conductor?
Current flows out and back, so the drop occurs over twice the one-way distance; battery and DC runs are especially sensitive.
How do I convert AWG to diameter?
d(mm) = 0.127 × 92^((36 − AWG)/39); each 3 gauge steps halves the area.
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This calculator is for educational purposes. Double-check important results before acting on them.