Wire Resistance Calculator gives the DC resistance of copper, aluminum and other wire from AWG, kcmil, mm² or diameter and length, corrected for temperature, with ohms per 1000 ft.
One bare conductor. For 1/0 to 4/0 wire, switch the size unit to /0 and type 1 to 4.
Temperature and Load
Wire Resistance Chart
| Size | Diameter | Per 1000 ft | Per km |
|---|---|---|---|
| 40 AWG | 0.07987 mm | 1.049 kΩ | 3.441 kΩ |
| 38 AWG | 0.1007 mm | 659.6 Ω | 2.164 kΩ |
| 36 AWG | 0.1270 mm | 414.8 Ω | 1.361 kΩ |
| 34 AWG | 0.1601 mm | 260.9 Ω | 856.0 Ω |
| 32 AWG | 0.2019 mm | 164.1 Ω | 538.3 Ω |
| 30 AWG | 0.2546 mm | 103.2 Ω | 338.5 Ω |
| 28 AWG | 0.3211 mm | 64.90 Ω | 212.9 Ω |
| 26 AWG | 0.4049 mm | 40.81 Ω | 133.9 Ω |
| 24 AWG | 0.5106 mm | 25.67 Ω | 84.21 Ω |
| 22 AWG | 0.6438 mm | 16.14 Ω | 52.96 Ω |
| 20 AWG | 0.8118 mm | 10.15 Ω | 33.31 Ω |
| 18 AWG | 1.024 mm | 6.385 Ω | 20.95 Ω |
| 16 AWG | 1.291 mm | 4.015 Ω | 13.17 Ω |
| 14 AWG | 1.628 mm | 2.525 Ω | 8.285 Ω |
| 12 AWG | 2.053 mm | 1.588 Ω | 5.211 Ω |
| 10 AWG | 2.588 mm | 0.9988 Ω | 3.277 Ω |
| 8 AWG | 3.264 mm | 0.6282 Ω | 2.061 Ω |
| 6 AWG | 4.115 mm | 0.3951 Ω | 1.296 Ω |
| 4 AWG | 5.189 mm | 0.2485 Ω | 0.8152 Ω |
| 3 AWG | 5.827 mm | 0.1970 Ω | 0.6464 Ω |
| 2 AWG | 6.544 mm | 0.1563 Ω | 0.5127 Ω |
| 1 AWG | 7.348 mm | 0.1239 Ω | 0.4066 Ω |
| 1/0 AWG | 8.251 mm | 98.27 mΩ | 0.3224 Ω |
| 2/0 AWG | 9.266 mm | 77.93 mΩ | 0.2557 Ω |
| 3/0 AWG | 10.40 mm | 61.80 mΩ | 0.2028 Ω |
| 4/0 AWG | 11.68 mm | 49.01 mΩ | 0.1608 Ω |
Step-by-Step Working
- Resistivity of annealed copper at 20 °C: ρ₂₀ = 1.7241 × 10⁻⁸ Ω·m, with α = 0.00393 per °C.
- Temperature: 68 °F = 20 °C, so ρ = ρ₂₀ × [1 + α × (T - 20)] = 1.7241 × 10⁻⁸ × 1 = 1.7241 × 10⁻⁸ Ω·m.
- Area: 12 AWG has d = 0.127 mm × 92^((36 - 12) ÷ 39) = 2.053 mm, so A = π × d² ÷ 4 = 3.309 mm².
- Length: L = 100 ft = 30.48 m.
- R = ρ × L ÷ A = 1.7241 × 10⁻⁸ × 30.48 ÷ 3.3088 × 10⁻⁶ = 0.1588 Ω.
Wire Resistance Calculator Formula and Inputs
The Wire Resistance Calculator works from one relationship. Resistance rises with the length of the conductor and the resistivity of its metal, and it falls as the cross-section gets larger.
R is the resistance in ohms, rho is the resistivity of the metal in ohm-meters, L is the length in meters and A is the cross-sectional area in square meters. That is how wire resistance is calculated for any metal and any size.
The tool opens on 100 ft of 12 AWG annealed copper at 68°F. Annealed copper has a resistivity of 1.7241 × 10⁻⁸ Ω·m at that temperature, the standard value in the NBS Copper Wire Tables, and 12 AWG has an area of 3.309 mm².
Wire size can be typed as an AWG number, an aught size from 1/0 to 4/0, kcmil, square millimeters or a diameter in millimeters or inches. That lets it serve as a wire resistance calculator for AWG and mm2 sizes alike, with the matching diameter, area and circular mils shown under the result.
Used as an electrical wire resistance calculator, leave the metal on annealed copper, the soft grade those tables are built on. Hard-drawn copper runs about 2.7% higher in resistivity, and Custom Resistivity can hold that value.
The answer is for one bare conductor of the length entered. A two-wire circuit sends current out and back, so its loop resistance is twice the figure for the one-way distance, and the load card shows that doubled drop.
Solve For changes what comes back. Besides resistance in ohms, the tool can find the length of a wire from a measured resistance, or the smallest standard size that stays under a target resistance.
Ohms per 1000 Feet and per Foot by AWG
The first result card gives ohms per 1000 ft and per kilometer for the size entered. Ohms per 1000 ft and milliohms per foot are the same number, so 12 AWG copper at 1.588 Ω per 1000 ft is 1.588 mΩ, or 0.001588 Ω, per foot.
The figures below are for solid annealed copper at 68°F (20°C), worked with the same math the Wire Resistance Calculator runs. Read the 1000 ft column in milliohms for the value per foot.
| Wire Size | Diameter | Ohms per 1000 ft | Ohms per km |
|---|---|---|---|
| 22 AWG | 0.644 mm | 16.14 Ω | 52.96 Ω |
| 20 AWG | 0.812 mm | 10.15 Ω | 33.31 Ω |
| 18 AWG | 1.024 mm | 6.385 Ω | 20.95 Ω |
| 16 AWG | 1.291 mm | 4.015 Ω | 13.17 Ω |
| 14 AWG | 1.628 mm | 2.525 Ω | 8.285 Ω |
| 12 AWG | 2.053 mm | 1.588 Ω | 5.211 Ω |
| 10 AWG | 2.588 mm | 0.9988 Ω | 3.277 Ω |
| 8 AWG | 3.264 mm | 0.6282 Ω | 2.061 Ω |
| 6 AWG | 4.115 mm | 0.3951 Ω | 1.296 Ω |
| 4 AWG | 5.189 mm | 0.2485 Ω | 0.8152 Ω |
| 1 AWG | 7.348 mm | 0.1239 Ω | 0.4066 Ω |
| 4/0 AWG | 11.68 mm | 0.04901 Ω | 0.1608 Ω |
Read per foot, the resistance of 20 gauge wire is 10.15 mΩ and the resistance of 22 gauge wire is 16.14 mΩ. The resistance of 100 ft of 1 AWG is 0.01239 Ω, or 12.39 mΩ. Going up three gauge numbers roughly doubles the resistance.
Open Wire Resistance Chart under the cards to see every size from 40 AWG to 4/0 at your own metal and temperature, with the entered size highlighted. For a wire resistance chart in ohms per foot as a PDF, press Print Results and choose Save as PDF, and the chart prints fully opened.
At 167°F (75°C) the result for 12 AWG is 1.932 Ω per 1000 ft, in line with the 1.93 Ω that NEC Chapter 9 Table 8 lists for solid uncoated copper. Stranded 12 AWG is listed slightly higher at 1.98 Ω, since each strand follows a spiral that is longer than the cable itself.
Resistance of 1.5, 2.5, 4 and 10 mm² Cable
To calculate cable resistance per meter, switch the size unit in the Wire Resistance Calculator to mm² and the length to meters. For annealed copper at 20°C the rule is short, since ohms per kilometer equal 17.241 divided by the area in mm².
| Size | Calculated, mΩ per m | Finolex Maximum, mΩ per m |
|---|---|---|
| 0.5 mm² | 34.48 | Not listed |
| 1.5 mm² | 11.49 | 13.3 |
| 2.5 mm² | 6.896 | 7.98 |
| 4 mm² | 4.310 | 4.95 |
| 6 mm² | 2.873 | 3.30 |
| 10 mm² | 1.724 | Not listed |
So the resistance of 2.5 mm cable per meter is about 6.9 mΩ, a 4 mm cable is 4.31 mΩ and a 1.5 mm cable is 11.49 mΩ. The resistance per meter of 10 mm cable is 1.724 mΩ, and a 0.5 sq mm wire comes to 34.48 mΩ. Milliohms per meter and ohms per kilometer are the same number.
Real stranded cable reads higher than the calculated column. The Finolex FR data sheet gives the maximum conductor resistance at 20°C for its flexible wire, and those limits sit about 15% above the solid-wire math. A sound coil should measure between the two columns.
That gap is why a stranded wire resistance calculator and a solid one can disagree. Strand count does not enter the formula. What changes is the true copper area and the extra length of the spiraled strands, so check a stranded cable against its own data sheet.
When a drawing mixes gauge and metric sizes, the AWG to mm² Calculator converts between them. A 2.5 mm² wire falls between 14 AWG and 12 AWG, which matches its place between 8.285 Ω and 5.211 Ω per kilometer in the gauge table.
Temperature, Metal and AC Frequency
Resistance climbs as a wire warms up. The correction sits under Temperature and Load, which makes this a wire resistance calculator with temperature built in. Near everyday temperatures the change is linear from the 20°C value.
Alpha is the temperature coefficient of the metal per °C and T is the wire temperature in °C. For annealed copper alpha is 0.00393, so resistance grows about 0.39% for each °C, or 2.18% for each 10°F.
The second card applies it at the conductor ratings used in wiring. The 100 ft of 12 AWG that reads 0.1588 Ω at 68°F becomes 0.1932 Ω at 167°F (75°C) and 0.2025 Ω at 194°F (90°C), a rise of about 22% and 28%.
The metals in the menu use the resistivity and coefficient values in the HyperPhysics resistivity table from Georgia State University. The last column is the resistance of 100 ft of 12 AWG in each metal at 68°F.
| Metal | Resistivity at 20°C, ×10⁻⁸ Ω·m | Coefficient per °C | 100 ft of 12 AWG |
|---|---|---|---|
| Silver | 1.59 | 0.0038 | 0.1465 Ω |
| Copper, pure | 1.68 | 0.00386 | 0.1548 Ω |
| Copper, annealed | 1.7241 | 0.00393 | 0.1588 Ω |
| Aluminum | 2.65 | 0.00429 | 0.2441 Ω |
| Tungsten | 5.6 | 0.0045 | 0.5159 Ω |
| Platinum | 10.6 | 0.003927 | 0.9765 Ω |
| Manganin | 48.2 | 0.000002 | 4.440 Ω |
| Nichrome | 100 | 0.0004 | 9.212 Ω |
In the Wire Resistance Calculator, aluminum has 1.54 times the resistance of annealed copper at the same size. Nichrome is 58 times higher, which is why it is wound into heating elements, and manganin hardly moves with temperature. Custom Resistivity accepts any metal that is not in the list.
The third card in the Wire Resistance Calculator reports conductance, which is 1 divided by the resistance, and the conductivity of the metal, 58.00 MS/m for annealed copper. Its last row is the frequency where skin effect lifts AC resistance 2% above the DC value in a solid round wire.
For 12 AWG copper that point is 4.06 kHz, far above power frequency. For 4/0 AWG it is about 125 Hz and for a 500 kcmil conductor about 53 Hz, so the largest conductors already read a little above their DC resistance at 60 Hz.
Wire Length or Size From a Resistance
Set Solve For to Wire Length and enter a measured resistance to find how much wire is on a spool or hidden in a wall. To calculate wire length using resistance, the Wire Resistance Calculator rearranges the first formula.
A reading of 0.5 Ω on 12 AWG copper works out to 314.8 ft of conductor. If the far end of a two-conductor cable is shorted and both wires are in the measurement, the cable itself is half that, 157.4 ft, and the line under the result shows both.
Wire Size mode goes the other way. Asking for no more than 0.5 Ω over 100 ft needs 1.051 mm² of copper, and the tool returns 16 AWG, the next standard size up, which gives 0.4015 Ω. Size Standard switches that answer to metric mm² sizes.
Enter the Load Current to see what the resistance costs in an electrical circuit. At 15 A the 100 ft of 12 AWG drops 2.382 V and turns 35.73 W into heat, and the out-and-back figure for a two-wire run is 4.765 V.
For sizing a circuit to a voltage drop limit and an ampacity table, the Wire Size Calculator runs both checks. The Voltage Drop Calculator tests a size you already have, and the Resistivity Calculator covers the material side of the same formula.
Checking Wire Resistance With a Meter
How to check resistance in a wire?
Switch the power off and disconnect at least one end of the wire so nothing else is in the measurement. Set the multimeter to its lowest ohms range, touch the probes together and note that reading. Then measure the wire end to end and subtract the lead reading.
Fluke states that test lead resistance should be 0.5 Ω or less, and that a relative (REL) function can store and subtract it. Most building wire runs are well under 1 Ω, so the leads can read more than the wire. The alert under the results flags any answer below 1 Ω for this reason and points to a four-wire Kelvin measurement.
What is a good resistance reading for a wire?
A good reading is one that matches the value from the Wire Resistance Calculator for that size, length and temperature. For 100 ft of 12 AWG copper that is 0.159 Ω, and for 100 ft of 14 AWG it is 0.2525 Ω. A reading well above the calculated figure points to a loose splice, corrosion, a damaged conductor or a longer route than assumed.
Is 3 ohms high resistance?
For a power conductor it is. Three ohms equals about 1,889 ft of 12 AWG copper, and at 15 A it would drop 45 V. How many ohms is too much for a wire depends on the current. On a 120 V, 15 A circuit held to a 3% drop, the whole out-and-back loop can be only 0.24 Ω, which is about 62 ft of 12 AWG each way at 167°F.
For a signal wire, a sensor lead or a heating element, 3 Ω can be normal. High resistance in wiring means resistance above what the conductor alone should have. The excess usually sits at a connection, where it turns into heat.
What is 1 Ω resistance?
One ohm is the resistance that lets 1 A flow when 1 V is applied. In wire terms that is a long run. The first card shows where each size reaches it, which is 629.6 ft for 12 AWG copper, 396 ft for 14 AWG and 580 m for 10 mm² cable.
Amps and Weight of Metric Wire
How many amps is 4 mm wire?
Resistance does not set the amp rating on its own, so the Wire Resistance Calculator stops at ohms, volts and watts. Ampacity depends on how hot the insulation may run, so it comes from the maker or the code table. Finolex rates its single-core copper wire in conduit at 13 A for 1.5 mm², 18 A for 2.5 mm², 24 A for 4 mm² and 31 A for 6 mm². Clipped in the open the same sizes carry 16 A, 22 A, 29 A and 37 A.
On those figures a 2.5 mm cable can take 16 A, while a 1.5 mm cable cannot take 16 A or 20 A in conduit. A 6 mm cable does not handle 40 A either, so that load moves up to 10 mm². For the load capacity of 10 mm wire, a useful marker is 8 AWG, slightly smaller at 8.37 mm², which NEC Table 310.16 lists at 40 A in the 60°C column and 50 A at 75°C.
A 1.5 ton AC drawing about 7.8 A at 230 V can run on 2.5 mm wire. Over a 20 m run, the 40 m of conductor has 0.276 Ω at 20°C and drops 2.15 V, under 1% of the supply. The Wire Ampacity Calculator looks up ratings by size and insulation.
What is the weight of a 1.5 sq mm copper wire per meter?
Standard annealed copper weighs 8.89 g per cm³, so each mm² of area weighs 8.89 g per meter of length. A 1.5 sq mm wire holds 13.3 g of copper per meter, 2.5 mm² holds 22.2 g and 4 mm² holds 35.6 g, before insulation is added.
Turned around, 1 kg of copper wire is about 75 m of 1.5 mm², 45 m of 2.5 mm² or 28 m of 4 mm² conductor. The price of 4 mm wire or 10 sq mm copper wire follows its copper content, 35.6 g and 88.9 g per meter, priced at the day's metal rate.