DC Voltage Drop Calculator
This calculator finds the voltage lost in the wires between a source and its load using Ohm's law, V equals I times R. Enter the load current with either the total round-trip resistance of the run or the resistance per meter and the one-way length, and it returns the volts dropped. Add a source voltage and it also reports the drop as a percentage and the voltage that actually reaches the load.
How to use the voltage drop calculator
- Pick how you know resistance in the 'Resistance from' menu: 'Per meter and length' or 'Total resistance'.
- Type the load current in amperes into the 'Load current' field.
- For per-meter mode, enter 'Resistance / meter' in ohm/m and the one-way 'Run length'; for total mode, enter 'Total resistance' in ohm.
- Optionally fill in 'Source voltage' to unlock the percent drop and end voltage.
- Read the Voltage drop, Percent drop and End voltage cards, then use Copy to grab all three lines.
Examples
Known total resistance, no source voltage
Total resistance mode: I = 10 A, total R = 0.5 ohm
Voltage drop = 5 V (percent and end voltage blank)
Per-meter over a 25 m run on a 120 V supply
Per meter mode: I = 10 A, 0.01 ohm/m, length 25 m, source 120 V
Voltage drop = 5 V, Percent drop = 4.1667%, End voltage = 115 V
16 A branch on a 230 V supply over 40 m
Per meter mode: I = 16 A, 0.005 ohm/m, length 40 m, source 230 V
Voltage drop = 6.4 V, Percent drop = 2.7826%, End voltage = 223.6 V
Frequently asked questions
How is DC voltage drop calculated here?
The drop equals the load current times the total conductor resistance, straight from Ohm's law V = I times R. For 10 A flowing through 0.5 ohm of wire the drop is 5 V, regardless of how that resistance was entered.
Why does per-meter mode double the length?
Current has to travel out to the load and return along the second conductor, so both legs add resistance. The tool builds the total as resistance per meter times 2 times the one-way length, which is why a 25 m run at 0.01 ohm/m gives 0.5 ohm, not 0.25.
What does the source voltage field change?
It is optional. Leave it blank and you still get the volts dropped, but the Percent drop and End voltage cards show a dash. Enter it and the tool reports drop divided by source times 100, plus source minus drop as the voltage left at the load.
What counts as an acceptable voltage drop?
Common practice keeps a branch circuit under about 3 percent and the whole feeder-plus-branch path under about 5 percent, so lights and motors get enough voltage. These are guidelines, so always defer to the wiring code and equipment specs for your installation.
Where do I get resistance per meter for my wire?
Look up the conductor in a wire table for its gauge and material. Copper and aluminum tables list ohms per 1000 feet or per kilometer at a reference temperature; divide to reach ohms per meter for one conductor before entering it.
Does this handle AC circuits or only DC?
It models resistive DC drop only. Real AC runs can add inductive reactance and depend on power factor and conductor spacing, none of which this calculator includes, so treat it as a close approximation for DC and low-frequency resistive loads.
Does conductor temperature affect the result?
Yes, in reality it does, because copper and aluminum resistance rises as the wire heats. This tool uses exactly the resistance you type, so if you need a hot-conductor figure, enter a resistance value already corrected for the expected operating temperature.
Are there input limits or units to watch?
Every value must be a positive number in base SI units: amperes, ohms, ohms per meter, meters and volts. Zero or negative entries are rejected with a prompt, and the source voltage is the only field you may leave empty.
Is my data sent anywhere?
No. The math runs entirely in your browser with no network request, so the currents, lengths and voltages you type never leave your device.
Learn more
- Sizing wire to control voltage drop
How run length, current and conductor size drive voltage drop, why the round trip matters, and practical ways to bring an over-limit run back into range.
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