Automotive Voltage Drop Calculator

Voltage drop is current × total circuit resistance. The tool estimates copper-wire resistance from AWG and round-trip length, then adds the connection resistance you enter. That makes it useful for comparing a marginal long circuit to a shorter/heavier relay-fed circuit.

For diagnosis, a meter test on the actual powered circuit is better than a calculated value. Put the meter across the section you want to test while the load is operating; the reading is the voltage being lost across that section.

A fuel pump can behave normally at idle and lose delivered flow when a weak supply, relay, connector or ground drops voltage under high current. Calculating expected wire drop gives you a sanity check; measuring the real drop shows whether terminals and grounds are adding resistance.

Wire ampacity depends on insulation, bundling, ambient temperature, installation and standards—not only voltage drop. This tool is not a fuse-sizing or ampacity approval. High-current starter circuits and safety-critical wiring should follow appropriate automotive electrical practices and component-manufacturer requirements.