Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Voltage drop (%)
6.58
Voltage drop (V)
7.90
Voltage at load
112.1
Recommended AWG for 3% drop
8
Power lost in wire (W)
158.0
Required circular mils
14333
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter load current (A).
- 2Enter system voltage (V).
- 3Enter one-way run length (ft).
- 4Enter wire gauge (awg).
- 5Enter conductor.
- 6Read your voltage drop (%) on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Keep voltage drop under 3% on branch circuits. Long runs to a shed or well pump are where undersized copper quietly costs you performance.
Worked example
Using the values the calculator loads with:
Inputs
- Load current: 20 A
- System voltage: 120 V
- One-way run length: 100 ft
- Wire gauge (AWG): 12
- Conductor: Copper
Results
- Voltage drop (%): 6.58
- Voltage drop (V): 7.9
- Voltage at load: 112.1
- Recommended AWG for 3% drop: 8
- Power lost in wire (W): 158
- Required circular mils: 14,333
What each field means
Inputs
- Load current (A)
- The load current used in the calculation, measured in A. Starts at 20 A so you have a working example on load.
- 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.
- 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 gauge (AWG)
- The wire gauge (awg) used in the calculation. Starts at 12 so you have a working example on load. Accepted range: 0–14.
- Conductor
- Pick the option that matches your situation — the maths changes per option. Choices: Copper, Aluminum.
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.
- Voltage 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.
- Recommended AWG for 3% drop
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Power lost in wire (W)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Required circular mils
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the wire size & voltage drop calculator work?
Keep voltage drop under 3% on branch circuits. Long runs to a shed or well pump are where undersized copper quietly costs you performance. The underlying maths is: Vdrop = 2 × K × I × L ÷ circular mils (K ≈ 12.9 for copper).
What do I need to enter?
5 values: load current, system voltage, one-way run length, wire gauge (awg), and conductor. 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. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Wire Gauge & Voltage Drop Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/wire-gauge
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/wire-gauge" target="_blank" rel="noopener">Wire Gauge & Voltage Drop Calculator — RevenueLab</a> (2026).</p>
Source: [Wire Gauge & Voltage Drop Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/wire-gauge) (2026).