Enter your circuit’s voltage, current, wire length and gauge to get the voltage drop — in volts and as a percentage.
What Is Voltage Drop?
A voltage drop calculator finds the volts lost in the wire: 12 V, 10 A, 50 ft one-way on 12 AWG copper → drop 1.59 V = 13.23% — way over the 5% guideline, so thicker wire is needed. Current flows out AND back, hence the ×2. Long runs at low voltage (12 V solar, RV, automotive) are where drop bites hardest.
How to Use
- Enter the source voltage.
- Enter the current in amps.
- Enter the one-way wire length.
- Pick the AWG gauge.
- Click “Calculate Drop”.
Formula
Drop = 2 × I × L × (Ω/1000ft ÷ 1000). % = 100 × drop ÷ V.
Worked Example
12 V, 10 A, 50 ft, 12 AWG (1.588 Ω/1000ft): drop = 2 × 10 × 50 × 0.001588 = 1.59 V = 13.23% — over the 5% guideline.
How It Works, In Plain Words
Work the formula in the order the page states it. The gauge list supplies resistance per 1,000 feet — 1.588 Ω/1000ft for the 12 AWG in the example — so dividing by 1,000 gives resistance per foot: 0.001588 Ω. Multiply by the one-way length (50 ft), by the current (10 A), and by 2, because the current makes the trip twice — out on one conductor and back on the other, each dropping voltage, exactly as the ×2 FAQ explains. That gives 2 × 10 × 50 × 0.001588 = 1.59 V. The percentage is the same loss measured against the source: 100 × 1.59 ÷ 12 = 13.23%.
The percentage is the number to judge, because the same 1.59 V means different things at different voltages — at 230 V it would be a footnote, at 12 V it leaves the load about 10.41 V of the 12 V it was promised, which is why the page says drop bites hardest at 12/24 V. Past 5% the tool appends its warning — “over the 5% guideline, use thicker wire” — and the acceptable-drop FAQ sets the frame: under 3% for branch circuits, 5% total. Safety note: this calculator is a planning guide for low-voltage runs. Real wiring work — fixed installations, mains voltages, anything connected to a distribution board — belongs to a qualified electrician; have the design checked before you buy cable, not after you bury it.
Real-World Uses
The worked example is the solar case the What Is section names: 12 V, 10 A over 50 ft on 12 AWG loses 13.23% — a panel run that needs the thicker wire the warning prescribes, checkable by stepping up to 10 AWG at 1.018 Ω/1000ft. An RV owner wiring a 12 V fridge run does the same check before choosing cable, because a fridge that sees far less than 12 V cycles badly. A boat owner with a long run to a 24 V load uses the percentage view: the FAQ’s point is that 24 V forgives what 12 V does not, and the tool shows by how much.
Benefits
- Drop in volts and percent
- The 5% guideline warning built in
- Copper AWG resistances built in
- Explains the ×2 round trip
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device.
Common Mistakes
- Entering the round-trip length — the tool doubles the one-way length itself
- Ignoring drop on low-voltage DC runs (where it matters most)
- Using aluminium resistances for copper wire (aluminium is ~1.6× higher).
Common Mistakes to Avoid
The list covers entering the round-trip length, ignoring drop on low-voltage DC runs, and using aluminium resistances for copper wire. In prose: do not judge by volts alone — 1.59 V sounds trivial until it is divided by 12 V and becomes 13.23%. Do not treat the 5% line as a target to land just under; the FAQ’s tighter figure is 3% for branch circuits. And do not fix a bad percentage by entering a hoped-for current instead of the load’s real draw: drop scales directly with amps, so an honest current is the whole calculation.
FAQ
What is acceptable voltage drop?
Under 3% for branch circuits, 5% total — the tool warns past 5%.
Why ×2 the length?
Current travels out and back — both conductors drop voltage.
Does voltage drop matter at 230 V?
Less — the same volts lost are a smaller percentage; it bites hardest at 12/24 V.
How do I reduce voltage drop?
Thicker wire (lower AWG), shorter runs, or higher system voltage.
Is the calculation for copper?
Yes — the built-in resistances are for copper; aluminium drops ~1.6× more.
What voltage does the load actually receive in the worked example?
About 10.41 V. The source supplies 12 V and the wire takes 1.59 V, so 12 − 1.59 = 10.41 V reaches the load — the practical meaning of the 13.23% figure. It is also why the warning fires: a load designed for 12 V running on roughly 10.41 V is being under-fed by more than the 5% guideline allows.
Can I use this for an aluminium cable run?
Only as a rough estimate, and expect worse. The gauge list is copper, and the page states aluminium’s resistance is ~1.6× higher, so the same run in aluminium drops roughly 1.6 times the volts the tool shows — the copper FAQ puts the drop at about 1.6× more. For a real aluminium design, use figures for the actual cable and have the work checked by a qualified electrician.