Voltage Drop Calculator
Enter the current, wire length, resistivity, and cross-section to get the DC voltage drop in volts — plus the power lost as heat — and see why thicker, shorter cable matters.
Drop and loss together
Enter the current, length, resistivity, and cross-section and the calculator returns the voltage drop (2LIρ/A) in volts and the power loss (Vdrop × I) in watts at once.
Use SI units
Amperes, metres, ohm-metres, and square metres give the drop in volts — 1 mm² of cross-section is 0.000001 m², so a 2 mm² wire is 0.000002 m².
What is voltage drop?
The voltage a cable steals
This voltage drop calculator turns four measurements — the current, the one-way wire length, the conductor resistivity, and the cross-sectional area — into the volts lost along a DC cable run and the power dissipated as heat. Voltage drop is the voltage that disappears across a conductor's own resistance as current flows through it, leaving less to power the load at the far end. Every metre of wire has a little resistance, so the longer the run and the thinner the conductor, the more voltage is lost before it ever reaches the device. It is the number behind sagging LED strips, under-powered motors at the end of long feeders, and the rule of thumb that you upsize cable on long runs.
Enter the current, length, resistivity, and cross-section to get the voltage drop in volts and the power lost as heat instantly.
The DC voltage drop is two times the one-way length multiplied by the current and the resistivity, divided by the cross-sectional area; the power loss is the drop times the current.
Vdrop = 2 × L × I × ρ ÷ ATake a 10 A load on a 20 m one-way run of 2 mm² copper, where the resistivity ρ is 1.68e-8 Ω·m. The factor of 2 covers the round trip out and back: 2 × 20 × 10 × 1.68e-8 = 6.72e-6, and dividing by the 0.000002 m² cross-section gives a 3.36 V drop. The power lost as heat is that drop times the current, 3.36 × 10 = 33.6 W. Halve the cross-section and the drop doubles; double the length and it doubles again.
The formula is exact for direct current, but a few practical points are worth keeping in mind.
DC resistance only and consistent units
This calculator gives the DC voltage drop from the conductor's resistance alone. It does not model AC effects such as inductive reactance or the skin effect, which matter for large cables and high frequencies, nor temperature rise that raises resistivity above its room-temperature value. Keep your units consistent — amperes, metres, ohm-metres, and square metres — or the volts will be wrong: convert mm² to m² by multiplying by 0.000001 before you enter the cross-section, and use the one-way length, not the round-trip length.