Copper Wire Resistance Calculator
Estimate resistance using wire length, size, and temperature. Supports AWG, diameter, or cross-sectional area input.
Assumptions: annealed copper resistivity at 20°C = 1.724×10⁻⁸ Ω·m, temperature coefficient α = 0.00393/°C.
What this copper wire resistance calculator does
This copper wire resistance calculator helps you quickly estimate how much electrical resistance a copper conductor adds to your circuit. Resistance matters because it causes voltage drop, heat generation, and power loss. Whether you are wiring a battery system, building an electronics project, or checking cable performance in a control panel, knowing wire resistance lets you make better design decisions.
By entering wire length, size, and temperature, you get resistance at both 20°C and your operating temperature. If you include current, the calculator also estimates voltage drop and I²R power loss.
Formula used for copper wire resistance
The calculator uses the standard physics relationship:
R = ρ × L / A
- R = resistance (ohms, Ω)
- ρ = copper resistivity (Ω·m)
- L = conductor length (m)
- A = cross-sectional area (m²)
Then it adjusts for temperature using:
RT = R20 × [1 + α × (T − 20)]
This means copper resistance rises as temperature increases. In hot environments, voltage drop can be significantly higher than expected if you only use room-temperature values.
AWG conversion details
When you choose AWG, the tool converts gauge to diameter using the standard AWG formula, then calculates area from diameter. This keeps results consistent with common American wire gauge tables and makes it easy to compare AWG resistance values with engineering references.
How to use the calculator correctly
- Length: Enter actual cable run in meters or feet.
- Circuit path: Use round-trip for most practical voltage drop calculations.
- Wire size: Choose AWG, diameter in mm, or area in mm².
- Temperature: Use realistic operating conductor temperature, not just ambient air temperature.
- Current: Optional, but needed if you want voltage drop and power loss.
Worked example
Suppose you have a 10 m one-way run of 12 AWG copper carrying 15 A in a warm enclosure, with conductor temperature around 50°C.
- Set length to 10 m
- Set path to round-trip (effective 20 m)
- Select 12 AWG
- Set temperature to 50°C
- Set current to 15 A
The calculator returns total resistance, estimated voltage drop, and power loss. This tells you immediately if the cable is acceptable or if a thicker conductor would reduce losses.
Typical copper wire values (quick reference)
| Wire size | Approx. area (mm²) | Approx. resistance at 20°C (Ω/km) |
|---|---|---|
| 18 AWG | 0.82 | ~21.0 |
| 14 AWG | 2.08 | ~8.29 |
| 12 AWG | 3.31 | ~5.21 |
| 10 AWG | 5.26 | ~3.28 |
| 6 AWG | 13.3 | ~1.30 |
Practical design tips
1) Prioritize voltage drop, not just ampacity
A wire may be thermally safe yet still drop too much voltage for sensitive loads. Motor starts, inverter systems, and low-voltage DC setups are especially sensitive.
2) Temperature matters more than many people expect
Copper resistance at elevated temperature can be noticeably higher. If your cable runs through attic spaces, equipment rooms, or bundled conduits, include realistic temperature effects.
3) Keep runs short where possible
Resistance scales linearly with length. Halving length roughly halves resistance, voltage drop, and resistive heating losses.
4) Use larger conductors when efficiency is important
Going up a wire size can dramatically reduce losses in high-current circuits. Over long operating hours, the energy savings can offset the higher cable cost.
FAQ
Is this calculator for aluminum wire?
No. This page is specifically for copper wire resistance. Aluminum has higher resistivity and needs different constants.
Why does the tool ask for round-trip length?
Current must travel to the load and back. In most two-conductor circuits, both conductors contribute to total loop resistance.
Can I use this for PCB traces?
You can use the same physics, but PCB copper thickness and geometry are different. For PCB design, use a trace-width calculator tailored to copper weight and trace temperature rise.
Final note
This copper wire resistance calculator is ideal for quick planning, AWG resistance checks, and first-pass voltage drop estimation. For critical installations, always verify with applicable electrical codes, manufacturer data sheets, and a licensed professional where required.