Voltage drop calculator
Estimate voltage drop from the conductor resistance you enter—using an explicit outgoing-and-return path, without selecting a cable or making a code-compliance judgment.
Enter source-to-load length once. The calculation doubles it for the outgoing and return conductors.
Manual-resistance estimate only. No conductor sizing, ampacity, temperature correction, reactance, installation method, protection or compliance verdict.
Included
- Two-conductor DC and resistive single-phase paths
- Manual resistance per length in metric or foot-based units
- Voltage drop, percentage, load voltage, loop resistance and I²R loss
Not included
- Conductor-size recommendations or material presets
- Ampacity, temperature, reactance, installation method and protection
- Local voltage-drop limits or compliance verdicts
What this means
The entered length is one way. Current travels to the load and back, so this narrow two-conductor model uses a loop length of 2 × L. Total loop resistance is that loop length multiplied by the entered resistance per length.
Ohm's law then gives ΔV = I × R<sub>loop</sub>. The percentage is the drop divided by source voltage, and estimated load voltage is source voltage minus the drop.
The calculator never chooses a conductor. Use a resistance value that already reflects the conductor and temperature basis you intend to examine, then take real installation decisions to qualified local guidance.
Formula & worked example
Rloop = 2 × L × r ΔV = I × Rloop Drop % = ΔV ÷ Vsource × 100 Vload = Vsource − ΔV
24 V DC, 5 A, 10 m one-way length and 2 Ω/km conductor resistance
- Loop resistance
- 2 × 10 m × 0.002 Ω/m = 0.04 Ω
- Voltage drop
- 5 A × 0.04 Ω = 0.20 V
- Estimated load voltage
- 24.00 − 0.20 = 23.80 V
The entered resistance implies a 0.833% drop and 1 W of conductor loss at the stated current.
How this calculation works
The calculation follows the formulas, definitions and assumptions explained on this page. The references below support the method and any stated boundaries.
Official sources
- OpenStax Physics — Ohm's law and voltage drop — Public V = IR relationship used to calculate voltage drop across the entered loop resistance
- NIST — exact foot-to-meter relationship — The international foot is exactly 0.3048 metres for length normalization
- NIST — SI units for electric current — Volt, ampere and ohm relationships used for base-unit calculations