Voltage Drop Calculator

Estimate voltage drop, voltage at the load, and power loss based on wire size, one-way wire length, current, voltage, wire material, and circuit type.

Enter Your Wire Run


Presets are rough examples. Use your actual voltage, current, length, and wire size when possible.


Examples: 12V, 24V, 48V, 120V, 240V.


Use the expected running current of the load.


Use the distance from the power source to the load, not round-trip length.


This calculator estimates voltage drop only. It does not approve wire ampacity.


Aluminum has higher resistance than copper at the same gauge.


Most low-voltage DIY projects use DC or two-wire calculations.


3% is a common planning target, but your project may differ.


Optional. Used to estimate wasted energy cost.


Optional. Used for energy loss over time.


Your Results

Voltage drop

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Enter values and calculate.

Drop percentage

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Compared with your target.

Voltage at load

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Estimated voltage reaching the device.

Power loss

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Energy lost as heat in the wire.

This calculator estimates voltage drop only. It does not determine whether a wire is safe, code-compliant, properly protected by a fuse or breaker, or suitable for the installation.

How to Use This Voltage Drop Calculator

Enter the source voltage, load current, one-way wire length, wire size, wire material, and circuit type. The calculator estimates voltage drop, voltage at the load, percentage drop, and power loss.

For DC and most two-wire low-voltage projects, enter the one-way length from the battery or power supply to the load. The calculator automatically accounts for the out-and-back wire path.

Formula Used

Wire resistance = resistance per 1,000 ft × wire length factor ÷ 1,000
DC / two-wire voltage drop = amps × wire resistance × 2 × one-way length
Three-phase voltage drop = amps × wire resistance × 1.732 × one-way length
Voltage drop % = voltage drop ÷ source voltage × 100
Power loss = voltage drop × amps

Example Calculation

A 12V load drawing 10 amps through 20 feet of 14 AWG copper wire has a long round-trip path for the current.

If the voltage drop is too high, the device may run poorly, lights may dim, motors may struggle, or the wire may waste more energy as heat.

Common Voltage Drop Mistakes

  • Using one-way distance manually and then forgetting the return wire path.
  • Assuming low voltage is always low current.
  • Ignoring voltage drop on 12V and 24V systems, where every volt matters more.
  • Choosing wire only by voltage drop and ignoring ampacity, insulation, temperature, fuse size, and installation conditions.
  • Using extension cords, connectors, or terminals that are not rated for the current.
  • Assuming a device will work correctly when voltage at the load is below its required range.

Important Safety Note

Use this calculator as a planning tool only. Voltage drop is not the same as wire safety. A wire can have acceptable voltage drop and still be unsafe if it is overloaded, unfused, damaged, installed incorrectly, or used in the wrong environment.

For mains wiring, breaker panels, permanent circuits, extension cord heat, burning smells, sparks, exposed conductors, or code-sensitive work, stop and contact a qualified electrician.