Use the free Wire Resistance Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Why Wire Itself Has Resistance
People often treat wire as a perfect conductor, but every real wire resists current a little. That resistance turns some energy into heat and drops some voltage along the run. Over short distances the effect is tiny, but over long runs, or with thin wire and high current, it becomes the difference between a circuit that works and one that does not. This calculator gives you the resistance of a wire run from its gauge, material, length, and temperature.
The formula is resistivity times length divided by cross-sectional area. Copper has a resistivity of about 1.68 times 10 to the power of minus 8 ohm-metres at 20 degrees Celsius, and aluminium is higher, at about 2.82 times 10 to the power of minus 8. Thicker wire has more area, so it has less resistance. Longer wire has more resistance. The calculator handles all of that, plus the temperature correction, because hot wire resists more than cold wire.
How to Use the Calculator
Choose a wire gauge from the list. The list uses American Wire Gauge, where smaller numbers mean thicker wire: 10 AWG is much thicker than 18 AWG. Each entry shows its cross-section in square millimetres so you can compare with metric wire sizes. Then choose copper or aluminium. Copper is the usual choice for household and hobby wiring; aluminium is lighter and cheaper but needs thicker sizes for the same resistance.
Enter the one-way length in metres, then the temperature. Twenty degrees is the standard reference. If the wire will run in a hot attic at 60 degrees, enter 60, and the calculator will raise the resistance accordingly using the temperature coefficient: 0.00393 per degree for copper and 0.00403 for aluminium. The result shows total resistance, resistance per metre, and the loop resistance for the out-and-back path, which is what matters in most circuits.
A Worked Example
Take 10 metres of 14 AWG copper wire at 20 degrees. Its cross-section is 2.08 square millimetres, and the calculator gives about 0.081 ohms one way, or 0.161 ohms for the loop. At 10 amps, that loop drops about 1.6 volts. On a 12 volt system, that is more than 13 percent of the supply, which is far too much. On a 120 volt system, it is only about 1.3 percent, which is fine. The same wire can be perfect or terrible depending on the system voltage, which is why voltage drop always deserves a calculation.
Now try the same run in aluminium. The resistance is about 68 percent higher, so the loop is about 0.27 ohms and the drop at 10 amps is about 2.7 volts. To match copper, you would need to step up roughly two gauge sizes. That is the standard trade-off with aluminium: it saves weight and cost, but you pay in size.
Temperature Matters More Than People Think
Resistance rises roughly 0.4 percent per degree Celsius for copper. A wire that measures 0.08 ohms at 20 degrees measures about 0.093 ohms at 60 degrees, a rise of about 16 percent. In a hot engine bay, attic, or conduit in sunlight, that extra resistance adds extra voltage drop and extra heat, which raises the temperature further. Always calculate at the hottest temperature the wire will actually see, not at room temperature.
Also remember that this calculator gives the resistance of the conductor alone. Connections, terminals, and splices add resistance too, and a poor connection can add more resistance than metres of wire. If a circuit drops more voltage than the calculator predicts, check the connections first.
Common Mistakes to Avoid
The most common mistake is entering the loop length as the one-way length, or the other way around. Be clear about which one you typed. The calculator asks for one-way length and shows you both results, so you can check yourself. Another mistake is comparing AWG numbers backward: remember that 10 AWG is thicker than 14 AWG. A third mistake is forgetting temperature, which quietly understates resistance in every hot installation.
This tool is an educational estimator. It estimates conductor resistance from standard resistivity values; it does not certify an installation or guarantee code compliance. Check your local electrical codes and consult a licensed electrician before doing any electrical work.
Frequently Asked Questions
Why is loop resistance double the one-way value? Because current travels out through one conductor and back through the other, so it crosses twice the length of wire.
Can I use this for stranded wire? Yes, approximately. Stranded wire of the same gauge has nearly the same cross-section as solid wire, so the resistance is very close.
Does insulation affect resistance? No, insulation does not change conductor resistance, but it does affect how hot the wire can safely run, which is a separate ampacity question.
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