Voltage Drop Calculator
Calculate voltage drop across a DC cable run. Enter wire length, cross-section (AWG or mm²), current and material, get voltage drop, power loss and percentage instantly.
Last updated: May 2026
Enter wire length, gauge and current to calculate voltage drop.
Vdrop = 2 × ρ × L × I ÷ A (round-trip)
Why voltage drop matters
Every wire has resistance. Current flowing through that resistance causes a voltage drop, the load receives less voltage than the supply provides. In a 12 V LED strip installation with thin wire over 10 m, that drop can be 1-2 V, causing dimming at the far end. In automotive wiring, excessive drop can prevent a starter motor from cranking. The IEC guideline for fixed building installations is a maximum 3% drop from source to load. Pairing this check with the current and power tools in the Electronics Hub helps you size a cable run that stays within that limit.
The formula uses the round-trip length (2 × one-way): Vdrop = 2 × ρ × L × I ÷ A, where ρ is resistivity in Ω·mm²/m, L is one-way length in metres, I is current in amps and A is cross-section in mm².
Common applications
- LED strip lighting over long runs (12 V or 24 V)
- Solar panel cable sizing (low voltage, high current)
- Automotive accessories and trailer wiring
- Low-voltage outdoor lighting and garden circuits
- PCB trace resistance estimation (convert mm² to trace width)
Voltage drop on low-voltage DC runs
Every DC build eventually arrives here. You started at Ohm's Law to pin down the base numbers, then sized an LED series resistor with the LED resistor calculator, maybe split a reference with the voltage divider calculator, and confirmed the supply can handle the load with the watts, volts and amps calculator. You are here: checking whether the wiring itself steals enough voltage to matter. On a 230 V mains circuit a 0.5 V drop is negligible. On a 12 V LED strip run the same drop is a visible colour shift and dimming at the far end, which is exactly where it hurts. After this step the last piece is runtime: the battery life calculator converts the load current confirmed here into hours on a given pack. For strips or accessories on AC driver boards, the reactance tools in the AC sub-chain (starting at the impedance calculator) take over where this DC page leaves off. The practical wiring guide for LED strip installations is the LED strip wiring guide, which ties voltage drop sizing directly to strip layout decisions.
Low voltage is where drop bites hardest. The same loss that barely registers at 230 V will dim a 12 V LED strip and shift its colour temperature noticeably, which is why this calculator defaults to 12 V. On the 12 V strip and automotive runs I wire myself, I size the cable from this figure rather than by eye, because the eye does not notice a slow fade along a five metre run until it is built.
Frequently Asked Questions
Why does the calculator multiply length by 2?
Current flows from the supply, through the positive wire to the load, and back through the negative (return) wire to the supply. Both wires have resistance and both contribute to the total voltage drop. If you are running 10 m of cable from a 12 V battery to a load, the total wire length in the circuit is 20 m. Using the one-way length and forgetting to double it gives half the actual drop, a common error in LED strip and solar installations.
What is an acceptable voltage drop percentage?
IEC 60364 recommends a maximum 3% drop for fixed installations (lighting and power circuits). For sensitive electronics, 1-2% is a better target. Automotive: Society of Automotive Engineers (SAE) allows up to 10% for non-critical circuits, 3% for critical circuits. For 12 V LED strips, a 5% drop (0.6 V) is usually acceptable at the far end, more causes visible dimming. Higher-voltage systems (24 V, 48 V) tolerate more absolute drop while keeping the percentage within limits.
How do I reduce voltage drop without changing the wire gauge?
Three options: (1) Reduce run length, split one long run into two shorter runs fed from both ends. (2) Increase supply voltage, a 24 V system carries the same wattage at half the current, dropping voltage drop to one quarter for the same wire. (3) Add a booster or buck converter at the far end to compensate. Increasing wire gauge is usually the most practical and lowest-cost option for fixed installations.
Why does a small voltage drop ruin a 12 V LED strip when the same drop is fine at 230 V?
Percentage is what matters to the load, not absolute volts. A 0.6 V drop on a 230 V circuit is 0.26%, invisible to any lighting load. The same 0.6 V on a 12 V strip is a 5% drop, and LED forward voltage is sensitive enough that a 5% supply reduction shifts colour temperature and cuts brightness visibly, especially at the far end of a long run. That is also why 24 V strip systems are popular: the same wire, same current, same absolute drop, but now it is 2.5% instead of 5%, bringing it comfortably within the IEC 3% guideline. If you are dealing with AC driver circuits rather than DC strip runs, see the impedance calculator for reactive load effects.
Methodology and sources
This tool calculates the resistive voltage drop along a DC cable run from the conductor resistivity, the round-trip wire length, the current and the cross-sectional area, then derives the loop resistance, the power lost as heat and the drop as a percentage of supply voltage.
- Method: Vdrop = 2 × ρ × L × I / A, where ρ is resistivity (Ω·mm²/m), L is one-way length (m), I is current (A) and A is cross-section (mm²). The factor 2 accounts for the round trip (feed + return). Loop resistance R = 2 × ρ × L / A and power loss P = I² × R (Joule heating). Percentage = Vdrop / Vsupply × 100.
- Standards and sources: Ohm's law and the conductor resistance relation (R = ρ × L / A); copper ρ = 0.0172 Ω·mm²/m and aluminium ρ = 0.0282 Ω·mm²/m (standard engineering values near 20 °C). The 3% drop target referenced on this page follows the IEC 60364-5-52 informative guidance for fixed installations (lighting); AWG-to-mm² values follow the standard American Wire Gauge series.
- Assumptions and limits: Pure DC resistive drop only, no reactance, skin effect or temperature rise beyond the room-temperature resistivity used. Conductor resistance is assumed constant; real cable warms under load and resistance rises with it. The calculation does not size a conductor for ampacity or thermal limits, check those separately before relying on a wire gauge.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This calculator is a planning aid, not a substitute for a qualified electrician or your local wiring and building code. Always verify conductor sizing, ampacity and protection against the applicable standard before installing.
Next step in this workflow
Drop checked: now estimate how long your battery will last at this load.