Voltage Drop Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
100% FreeRuns in your browserIntermediate
Enter Values
Enter your values and press Calculate to see results.
How to use this calculator
- Enter the current, one-way cable length and conductor resistance (mΩ/m).
- Enter the supply voltage for the percentage.
How it works
Drop = 2 × length × current × resistance (the 2 accounts for the return path). Percentage = drop ÷ supply voltage.
Worked example
20 A, 30 m, 7.41 mΩ/m, 230 V. ≈ 8.9 V drop (3.87%).
Common mistakes
- Using the round-trip length — enter the one-way run; the formula doubles it.
- Guessing resistance — use the cable's rated mΩ/m for its size and material.
Frequently asked questions
What's an acceptable voltage drop?
Many standards aim for 5% or less total. This is a guide only and not a substitute for a licensed electrician's design.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.