Rex says
The fast lane for the math you almost remember from school. Type the numbers, get the answer, move on with your day.
Try a scenario
Click to load — tweak from there.Inputs
12 AWG ≈ 6530, 10 AWG ≈ 10380
Result
Voltage drop (%)
4.94
Drop (V)
5.93
Voltage at load
114.1
Verdict
Over 3% — upsize wire
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter system voltage (V).
- 2Enter load current (A).
- 3Enter one-way run length (ft).
- 4Enter wire area (circular mils) — 12 AWG ≈ 6530, 10 AWG ≈ 10380.
- 5Read your voltage drop (%) on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
Long runs bleed voltage. Codes generally want branch-circuit drop under 3%. This estimates drop from length, current, and copper wire area.
Worked example
Using the values the calculator loads with:
Inputs
- System voltage: 120 V
- Load current: 15 A
- One-way run length: 100 ft
- Wire area (circular mils): 6530
Results
- Voltage drop (%): 4.94
- Drop (V): 5.93
- Voltage at load: 114.1
- Verdict: Over 3% — upsize wire
What each field means
Inputs
- System voltage (V)
- The system voltage used in the calculation, measured in V. Starts at 120 V so you have a working example on load.
- Load current (A)
- The load current used in the calculation, measured in A. Starts at 15 A so you have a working example on load.
- One-way run length (ft)
- The one-way run length used in the calculation, measured in ft. Starts at 100 ft so you have a working example on load.
- Wire area (circular mils)
- 12 AWG ≈ 6530, 10 AWG ≈ 10380
Results
- Voltage drop (%)
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Drop (V)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Voltage at load
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Verdict
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the wire voltage drop calculator work?
Long runs bleed voltage. Codes generally want branch-circuit drop under 3%. This estimates drop from length, current, and copper wire area. The underlying maths is: Vdrop = 2 × K × I × L ÷ CM (K ≈ 12.9 for copper)
What do I need to enter?
4 values: system voltage, load current, one-way run length, and wire area (circular mils). Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the voltage drop (%) result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are rounded for display; the underlying calculation keeps full precision.
Who is this tool for?
It's built for students, developers, and anyone who needs the answer right now without setting up a spreadsheet.
Accuracy and limitations
- Results are rounded for display; the underlying calculation keeps full precision.
- Very large or very small inputs may hit floating-point limits in the browser.
- Inputs outside the accepted range are clamped rather than rejected.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Voltage Drop Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/voltage-drop
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/voltage-drop" target="_blank" rel="noopener">Voltage Drop Calculator — RevenueLab</a> (2026).</p>
Source: [Voltage Drop Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/voltage-drop) (2026).