Wire Voltage Drop Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Method from U.S. National Institute of Standards and Technology. Last verified 2026-06-23.

The Wire Voltage Drop Calculator computes wire voltage drop from the relation voltage drop = 2 x K x current x length / circular mils. It takes 4 inputs (resistivity constant k (copper 12.9, aluminum 21.2), load current in A, one way circuit length in ft, conductor area in circular mils in cmil) and returns the wire voltage drop. Because this is a pure mathematical or physical formula rather than a jurisdiction-specific rule, the result never changes over time: the same inputs always produce the same answer, so you can rely on it whether you are checking homework, sizing a design, or sanity-checking another tool. Enter your values in the fields below and the result updates instantly; you can also share a permalink that pre-fills the exact calculation, which is useful for teaching, reports, or collaboration. For example, with resistivity constant k (copper 12.9, aluminum 21.2) = 12.9, load current = 10 A, one way circuit length = 100 ft, conductor area in circular mils = 16510 cmil, the wire voltage drop works out to 1.562689, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. The method is the standard form documented by U.S. National Institute of Standards and Technology, and the figure above each result carries the date it was last verified. This tool is general information and is not a substitute for professional engineering, medical, financial, or scientific advice; always check critical results against the primary source and your own judgement.