Voltage Drop Calculator
Find voltage drop over a wire run.
For % drop
Results update as you type.
How it works
Voltage drop is the loss of voltage as current flows through the resistance of a wire, and this calculator finds it for a cable run. You enter the current, the one-way length of the run, the wire's resistance per kilometer, and optionally the supply voltage, and it returns the voltage drop in volts and, if you provide the supply, as a percentage. The calculation applies Ohm's law along the cable: voltage drop equals current times the total wire resistance, where the resistance is the per-kilometer figure scaled by the length. The length is doubled because current flows out along one conductor and back along another, so both add resistance. Voltage drop matters because excessive drop starves equipment of voltage, causing dim lights, overheating motors, malfunctioning electronics, and wasted energy as heat in the wire. Electrical codes typically limit voltage drop to around three percent for a branch circuit and five percent overall, which is why the percentage output is useful. Long runs, high currents, and thin wires all increase drop, so the fix is usually a larger conductor, a shorter route, or a higher system voltage. Electricians and engineers use this when sizing cables for buildings, solar arrays, and long feeders. Enter the current, length, and wire resistance to compute the voltage drop.
V_drop = 2 · (length / 1000) · R_per_km · I
Voltage drop equals twice the one-way run length in kilometers, times the wire's resistance per kilometer, times the current. The factor of two accounts for the current traveling out and back along both conductors. If a supply voltage is given, the drop is also expressed as a percentage of it, which codes typically limit to a few percent.
Source:Electrical engineering
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter current, one-way run length and wire resistance.
- 2
Adjust the options
Adjust supply voltage (optional) if they apply — sensible defaults are already set.
- 3
Read your result
Voltage drop updates instantly as you type — no button to press.
Frequently asked questions
How do I calculate voltage drop?
Multiply the current by the total round-trip wire resistance. The round-trip resistance is twice the one-way length in kilometers times the resistance per kilometer. For 10 amps over a 50-meter run at 8 ohms per kilometer, the drop is 8 volts. The calculator also shows this as a percentage if you enter the supply voltage.
Why is the length doubled?
Current flows from the source to the load along one conductor and returns along another, so it passes through wire twice the one-way distance. Both conductors add resistance and cause voltage drop, which is why the formula multiplies the one-way length by two. Ignoring this would underestimate the drop by half.
What is an acceptable voltage drop?
Electrical codes commonly recommend limiting voltage drop to about three percent on a branch circuit and five percent total from the source to the load. Staying within these limits ensures equipment gets adequate voltage and avoids energy waste. The percentage output helps you check your run against these guidelines when you enter the supply voltage.
How do I reduce voltage drop?
Use a larger, lower-resistance conductor, shorten the cable run, reduce the current, or raise the system voltage. Since drop is proportional to resistance, length, and current, reducing any of these helps, but increasing wire size is the most common fix for long runs. Higher voltages carry the same power at lower current, cutting drop.
Where do I find the wire's resistance per km?
Wire resistance per kilometer depends on the conductor material and cross-sectional area, and is listed in cable datasheets and standard wire tables by gauge or size. Copper has lower resistance than aluminum for the same size. Look up the value for your specific conductor and enter it for an accurate voltage drop.