Wire Voltage Drop Calculator
Calculate the voltage lost in a cable and the heat it produces from the wire size (AWG or mm²), length and current.
Formula and cautions
R = (ρ·L)/(A) and ΔV = I·R. Current flows out on one wire and back on the other, so the round-trip length is normally used. At low DC voltages such as 12 V or 5 V a small drop is a large share of the supply, which is why LED strips and motors often misbehave on long runs. This tool covers voltage drop only. How much current a wire can safely carry depends on its insulation and installation, so check the cable's ampacity rating separately.
Worked example: 12 V LED strip, 3 A, 5 m away
- AWG 18: 10 m round trip is 209 mΩ → drop 628 mV (5.2%)
- AWG 16: 10 m round trip is 132 mΩ → drop 395 mV (3.3%)
- AWG 14: 10 m round trip is 82.8 mΩ → drop 249 mV (2.1%)
- To stay under 3%, choose AWG 14.
AWG wire size chart
| AWG | Diameter (mm) | Area (mm²) | Resistance (Ω/km, copper 20 °C) |
|---|---|---|---|
| AWG 30 | 0.255 | 0.051 | 338.53 |
| AWG 28 | 0.321 | 0.081 | 212.90 |
| AWG 26 | 0.405 | 0.129 | 133.90 |
| AWG 24 | 0.511 | 0.205 | 84.21 |
| AWG 22 | 0.644 | 0.326 | 52.96 |
| AWG 20 | 0.812 | 0.518 | 33.31 |
| AWG 18 | 1.024 | 0.823 | 20.95 |
| AWG 16 | 1.291 | 1.309 | 13.17 |
| AWG 14 | 1.628 | 2.081 | 8.28 |
| AWG 12 | 2.053 | 3.309 | 5.21 |
| AWG 10 | 2.588 | 5.261 | 3.28 |
| AWG 8 | 3.264 | 8.366 | 2.06 |
Using it in practice
Step by step
- Choose copper or aluminium and enter the wire size as AWG or mm².
- Enter the length and the current, and tick round trip if the current goes out and back through the same length of cable.
- Read the voltage drop, the percentage of the supply it represents and the heat in the wire.
Common mistakes
- Counting only one conductor. A DC load 5 m away needs 10 m of wire, out on the positive lead and back on the negative. Leaving the round trip off halves the drop.
- Choosing wire by ampacity alone. A wire can be safe from overheating and still drop too much voltage over a long run, especially on 5 V and 12 V systems.
- Ignoring connectors. Screw terminals, crimp joints and cheap barrel jacks each add tens of milliohms. On a USB or 5 V LED strip, they can matter as much as the wire.
How much drop is acceptable
A common guideline is 3% for circuits that need a stable voltage and up to 5% for less critical loads. On low-voltage systems check the device's minimum input: a 5 V board that resets below 4.5 V tolerates only 0.5 V in total, wiring and connectors included. Aluminium has about 1.6 times the resistance of copper for the same size, so it needs a larger cross-section. Wire resistance also rises about 0.4% per °C, so hot cables drop more.
Worked example: a 12 V LED strip 5 m from the supply
The strip draws 3 A and is wired with 0.75 mm² copper. The current goes out and back, so the cable length is 10 m. R = (ρL)/(A) = 1.724 × 10⁻⁸ Ω·m × 10 m ÷ 0.75 mm² = 0.230 Ω. The drop is 3 A × 0.230 Ω = 0.69 V, which is 5.7% of 12 V, and the cable turns 2.07 W into heat.
Doubling the cross-section to 1.5 mm² halves everything: 0.115 Ω, 0.34 V (2.9%) and about 1 W. The far end of the strip now sits near 11.7 V instead of 11.3 V, which you can usually see as more even brightness. Feeding the strip from both ends does the same job when thicker wire is awkward.
Measuring the drop on the bench
Run the load at full current and measure the voltage at the supply terminals and at the load terminals; the difference is the total drop. Then measure along each lead separately, from the supply's positive terminal to the load's positive terminal and the same for the negative side. The two readings should be close to equal. If one is much larger, a connector, crimp or screw terminal on that side is the problem, not the wire gauge. A drop that grows while the cable warms up is the copper's temperature coefficient at work.
Frequently asked questions
How much voltage drop is acceptable?
A common guideline is 3% or less for a single run and 5% or less overall. Low-voltage DC loads such as LED strips and small motors benefit from staying near 3% or below.
How do AWG and mm² sizes compare?
AWG is the American wire gauge, where a smaller number means a thicker wire. Metric sizes give the copper cross-section in mm² directly. AWG 18 is about 0.82 mm², close to 0.75 mm² metric wire.