Wire Resistance Calculator
Choose the metal, the length and the size by area, diameter or AWG gauge. You get the resistance, and the voltage drop and heat at a given current.
Resistivity at 20 °C. Temperature correction is built in for copper and aluminium.
Worked examples
Copper cable
100 m of 2.5 mm2 copper at 20 C
0.672 ohms
1.68e-8 x 100 / 2.5e-6.
Hot cable
The same cable at 70 C
0.804 ohms
Multiplied by 1 + 0.00393 x 50.
About Wire Resistance Calculator
The resistance of a wire depends on what it is made of, how long it is and how thick it is. It equals the resistivity of the metal multiplied by the length and divided by the cross-section area. Doubling the length doubles the resistance, and doubling the area halves it. This calculator does the sum for copper, aluminium, silver, gold, tungsten, iron and nichrome, or for any resistivity you enter.
The size can be given as an area in square millimetres, a diameter, or an American Wire Gauge number, with the diameter found from the standard gauge formula. Resistance rises with temperature, so for copper and aluminium the page applies the standard temperature coefficients. Enter a current to see the voltage lost along the wire and the power turned into heat, which tells you if a thin wire will warm up.
How to use Wire Resistance Calculator
- Choose the material, or enter your own resistivity.
- Enter the length of wire.
- Choose how the size is given and enter it as an area, diameter or AWG number.
- Set the wire temperature and optionally a current, then read the resistance, voltage drop and heat.
Common uses
- Estimating the loss in a long extension lead or speaker cable
- Choosing a wire gauge for a low voltage lighting run
- Designing a heating element from nichrome wire
- Checking the resistance of a coil of wire
Frequently asked questions
What is the formula for the resistance of a wire?
R = rho x L / A, where rho is the resistivity of the material in ohm metres, L is the length in metres and A is the cross-section area in square metres.
How does temperature change resistance?
Metals have more resistance when hotter. For copper the resistance rises about 0.39% for each degree Celsius above 20, and for aluminium about 0.40%.
How do I convert AWG to millimetres?
The diameter in millimetres is 0.127 times 92 raised to the power of (36 minus the gauge number) divided by 39. AWG 18 is about 1.02 mm and AWG 10 about 2.59 mm.
Do I double the length for a circuit?
Yes. Current flows out and back, so the whole loop is twice the distance between the supply and the load.