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Voltage drop calculator

Estimate the voltage drop in a copper cable from its length, current and cross-section.

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Need Voltage drop (V), Voltage drop (%)? The Voltage drop calculator derives it from Current (A), Cable length (m, one way), Cross-section (mm²), System voltage (V) in one step. For instance, with Current (A) = 10, Cable length (m, one way) = 20, Cross-section (mm²) = 2.5 and System voltage (V) = 230 it returns Voltage drop (V) = 2.8 and Voltage drop (%) = 1.22%.

How to use it

  1. Enter your values: Current (A), Cable length (m, one way), Cross-section (mm²), System voltage (V).
  2. Read the result instantly: Voltage drop (V), Voltage drop (%).

Frequently asked questions

What does the Voltage drop calculator actually compute?

It takes Current (A), Cable length (m, one way), Cross-section (mm²) and System voltage (V) and derives Voltage drop (V) and Voltage drop (%) from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

4 values: Current (A), Cable length (m, one way), Cross-section (mm²) and System voltage (V). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Current (A) = 10, Cable length (m, one way) = 20, Cross-section (mm²) = 2.5 and System voltage (V) = 230, the calculator returns Voltage drop (V) = 2.8 and Voltage drop (%) = 1.22%. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Current (A) = 20, Cable length (m, one way) = 40, Cross-section (mm²) = 5 and System voltage (V) = 460 instead, Voltage drop (V) goes from 2.8 to 5.6 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Current (A) = 5, Cable length (m, one way) = 10, Cross-section (mm²) = 1.3 and System voltage (V) = 115, Voltage drop (V) comes out at 1.346. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Putting a number on a bill: what an appliance costs to run, whether a solar array covers the load, and how much heating a room actually needs.

What is the most common mistake?

Confusing power with energy. A kilowatt is a rate and a kilowatt-hour is a quantity; a 2 kW heater on for half an hour uses 1 kWh, and only the second figure appears on a bill.

What is the difference between the Voltage drop calculator and the Voltage divider calculator?

This one returns Voltage drop (V) and Voltage drop (%); the Voltage divider calculator returns Output voltage (V). That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Penny drop impact calculator is the closest one after this: Would a penny dropped from a skyscraper really kill someone? This tool applies quadratic air drag to find the terminal velocity a coin actually reaches, its true impact speed after any fall height, and the kinetic energy — debunking the famous myth with physics.

What else is worth having open alongside it?

Amperage Calculator and Heat pump savings calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
ExplainerWhat Is Voltage Drop? Why Long Wires Lose VoltageVoltage drop is the voltage lost along a wire due to its resistance. Learn how length, gauge, and current affect it, and why it matters for your circuits.ExplainerHow Many Cables Fit in a Conduit? It Is a Code Question, Not a Geometry QuestionThe 40 % limit for three or more conductors is not about whether they physically fit. It is about heat and about pulling them in without stripping the insulation — and the moment you use that space, the code takes current away from every conductor in the pipe.How-toHow to Calculate an LED Series ResistorSize the series resistor for any LED with one formula: R = (Vs − Vf) / I. Follow the steps, see a worked 5 V example, and pick the right resistor.ExplainerWhat Is Ohm's Law? V = I × R Explained SimplyOhm's law links voltage, current, and resistance with one equation: V = I × R. Learn the triangle trick and see worked examples.How-toHow Many Amps Does an Appliance Draw?Divide the appliance wattage by the supply voltage to get the current in amps — the number that decides breaker size, wire, and what can share a circuit.How-toWhat Wire Gauge Do I Need?Pick wire size from the load current, run length, and allowed voltage drop — the amps decide the minimum cross-section, distance forces you thicker.