Voltage Drop Calculator

Find the voltage lost across a circuit's resistance from current and resistance.

⏱ Updated: 17 Sep 2026

Calculator

Free voltage drop calculator: find the voltage lost across a circuit's resistance from current and resistance.

Voltage Drop
V
% of Source Voltage
%

Enter the total resistance of the wire or circuit run (which depends on wire gauge, length, and material — look this up separately for your specific wire). This calculator applies Ohm's Law (V = I × R) to that resistance.

Enter a current, circuit resistance, and source voltage to calculate the voltage drop.

How to Use the Voltage Drop Calculator

Current, resistance, and source voltage together show how much voltage a circuit loses over that resistance, and what's left downstream.

Every real wire has some resistance, and pushing current through that resistance costs voltage — the longer or thinner the run, the more you lose before it reaches whatever's on the other end. This Voltage Drop Calculator applies Ohm's Law to a circuit's resistance to find how much voltage disappears along the way, and what share of the source voltage that represents.

voltage drop = current × circuit resistance

The resistance value itself isn't something this tool looks up — it depends on wire gauge, length, and material, all of which vary by what you're actually running. Get that number from a wire gauge chart or a direct measurement, then let this handle the rest.