About the Wire Resistance Calculator
Every piece of wire resists the current, and the resistance grows with length and shrinks with thickness. This calculator works it out from the metal, the length and the diameter. If you also give a current it shows the voltage that is lost along the wire and the power that turns into heat, which is the reason to choose cable thick enough for the job. Type the metal, the length, the diameter and, optionally, the current and the wire resistance appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Wire Resistance Calculator
- 1Enter the metal, the length, the diameter and, optionally, the current.
- 2Pick the units or options from the menus if the wire resistance tool offers them.
- 3Read the wire resistance in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the wire resistance
Installers and hobbyists use it to check whether a long cable run, such as a garden light, a solar panel or a battery lead, will lose too much voltage. Makers use it to estimate the resistance of coils and heating wire.
Nichrome has a resistivity around 65 times that of copper, which is why it is used for heating elements. Silver conducts best, but copper is cheaper. Aluminium needs a thicker wire than copper for the same resistance.
How the wire resistance is worked out
R = resistivity × length ÷ cross-section area. Voltage drop = current × resistance, and heat loss = current² × resistance. A 10 m length of 1 mm copper wire has an area of 0.785 mm² and a resistance of 0.214 Ω. At 5 A the voltage drop is 1.07 V and the wire turns 5.35 W into heat. The same aluminium wire has 0.359 Ω.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Seven metals
- Voltage drop and heat loss
- Metres, feet, millimetres or inches
Mistakes to avoid with wire resistance
Resistivity values are for 20 °C. Copper's resistance rises about 0.4 percent for each degree of warming, so a hot cable carries slightly less. The calculation uses the length of the wire itself: a circuit has a go and a return conductor, so double the cable length for the round trip.
On phone and computer
On a phone, type into the boxes one after another and read the wire resistance underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the wire resistance move. The wire resistance calculator works the same on both. To see how much voltage a run loses use the Voltage Drop Calculator, and Ohm's Law Calculator works out the current.
Features
- Double the cable for a round trip
- Temperature raises resistance
- Thin wire gets hot
- Clear messages when the wire resistance inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate wire resistance?
R = resistivity × length ÷ cross-section area. Voltage drop = current × resistance, and heat loss = current² × resistance. A 10 m length of 1 mm copper wire has an area of 0.785 mm² and a resistance of 0.214 Ω. At 5 A the voltage drop is 1.07 V and the wire turns 5.35 W into heat. The same aluminium wire has 0.359 Ω.
What is the formula for wire resistance?
R = resistivity × length ÷ cross-section area. Voltage drop = current × resistance, and heat loss = current² × resistance. The wire resistance calculator applies it exactly and rounds only the displayed answer to six significant figures.
Why does a thin wire get hot?
Its resistance is higher, so the same current produces more heat: power = current² × resistance.
Do I use the one-way or round-trip length?
The full length of wire the current travels through, so double the cable length for a two-wire circuit.
What diameter is 18 AWG?
About 1.02 mm. Use the Wire Gauge Calculator to convert gauges to millimetres.
Is this wire resistance calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.