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Voltage Drop Calculator | Estimate Voltage Drop and Percentage

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Calculate voltage drop along a wire run from length, gauge, current and material — copper or aluminium — with the round-trip distance applied. Size wires correctly for safe, code-compliant electrical installations.

Last updated: June 2026 · Free · No sign-up required

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Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Copper resistance by wire size (approx, ohms/1000 ft):

AWGOhms/1000 ft
142.525
121.588
100.999
80.628
60.395

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Interpretation guide

Acceptable voltage drop:

GuidelineLimit
Branch circuit<= 3% recommended
Feeder + branch<= 5% total
Effect of long runsMore drop, may need larger wire
Effect of higher currentProportionally more drop

lightbulb Worked example — 50 ft run of 12 AWG at 15 A

A 120 V branch circuit: 50 ft one-way of 12 AWG copper (1.588 Ω per 1,000 ft), carrying 15 A. Voltage drop = 2 × 50 × 15 × 1.588 ÷ 1,000 = 2.382 V. Percent = 2.382 ÷ 120 × 100.

Result: 2.38 V drop — 1.99% of 120 V.

What this means: Under the 3% guideline for branch circuits, so 12 AWG is fine here. Stretch that run to 100 ft and the drop doubles to about 4% — then you'd step up to 10 AWG.

Formula & methodology

Formula: Vdrop = 2 x L x I x R / 1000 (single phase); % drop = Vdrop / Vsource x 100

How voltage drop is estimated

Drop = 2 × one-way length × current × resistance per unit length. The length doubles because current travels out and back on a single-phase circuit; the resistance comes from NEC Chapter 9, Table 8 values for the wire gauge and material.

  • Percent drop = drop ÷ source voltage × 100.
  • Guideline: ≤3% on branch circuits, ≤5% total from service to outlet.

Aluminum wire of the same gauge has about 61% of copper's conductivity, so it drops roughly 64% more voltage over the same run.

Authoritative source: timeanddate.com

Frequently asked questions

How do I calculate voltage drop?
Enter the required values into the input fields and press the calculate button. The tool applies the formula Vdrop = 2 x L x I x R / 1000 (single phase); % drop = Vdrop / Vsource x 100 and returns the result with a short explanation of each step, so you can check the working yourself.
What is an acceptable voltage drop?
The Voltage Drop Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Estimates the voltage drop along a wire run for electrical installations. Enter the conductor size, current in amps, wire length, and circuit voltage, and the tool calculates the voltage drop in volts and as a percentage, using standard conductor resistance There is no limit on usage, and it works on any device.
How do I reduce voltage drop?
The three levers are wire size, distance, and current. Increase the wire gauge (larger cross-section lowers resistance), shorten the run, or reduce the load. The formula shows resistance grows with length and shrinks with cross-sectional area, so upsizing the wire one or two gauges is usually the most practical fix for long runs.
Why multiply length by two?
Because current must travel out AND back — the circuit length is the full loop, so the one-way distance is doubled in the voltage-drop calculation. A 100-foot run from the panel to the outlet is 200 feet of conductor. This doubling is why voltage drop becomes significant on long runs, especially at low voltages.
Does aluminum wire drop more voltage than copper?
Yes — aluminum has about 61% of copper's conductivity, so for the same gauge, length, and current, aluminum wire drops about 64% more voltage (resistance is roughly 1.64×). To match copper's performance, aluminum needs to be upsized by about two gauge numbers. The calculator uses material-specific resistivity for both.
Does it handle three-phase circuits?
The calculator covers single-phase runs, where current travels out and back (hence the ×2 length). Three-phase drop uses a √3 factor instead of 2 with different conductor math — for three-phase work, treat the result as a rough guide and confirm with an electrician or the NEC tables.
Where do I find my wire's resistance per 1,000 feet?
From NEC Chapter 9, Table 8: 12 AWG copper is 1.588 Ω/1,000 ft, 10 AWG is 0.999 Ω, and aluminum runs higher for the same gauge. The calculator has common values built in — just pick your gauge and material. Using the wrong material's value is the most common error.
Is my data sent anywhere when I calculate?
No — the calculation runs on the page itself, so your wiring details never leave your device.
How precise is the estimate?
Very close for copper at room temperature — within a few percent of measured drop. Real wire runs hotter under load (copper's resistance rises ~0.4% per °C), so heavily loaded circuits drop slightly more than the room-temperature math says.

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