Speaker Cable Calculator

Free tool to calculate voltage drop and power loss in a speaker cable. No sign-up

Choose a cable size from the list, or type the cross-section in mm².
Enter the length of the run, from amplifier to speaker, in metres or feet.
Enter the speaker impedance (4 or 8 Ω are the most common) and the power of your amplifier into that load.
Choose the maximum loss you accept: 0.5 dB is a common choice.
Read the minimum section at the top, then the voltage and the power at the speaker, and check the table for other sizes.
Download the results as CSV or copy them as text.

Length is the run from the amplifier to the speaker: the return wire is counted for you. The power is what the amplifier delivers into this load with no cable; the tool shows how much of it reaches the speaker. Rows over the maximum loss are highlighted.

Nothing is saved or uploaded: everything stays in your browser. Download or copy your results before closing the page. The calculation uses copper at 20 °C, treats the amplifier as an ideal voltage source and the speaker as a constant nominal impedance. Real speakers change impedance with frequency, and cable capacitance, inductance and connector resistance are ignored, so use the result as an estimate.

How do you calculate the voltage drop and power loss of a speaker cable?

The resistance of a copper cable is R = 0.0172 × 2 × length ÷ cross-section, with the length in metres, the cross-section in mm² and the factor 2 counting the way out and the way back. This resistance is in series with the speaker, so the voltage that reaches it is V × Z ÷ (Z + R), and the power lost as heat is I² × R. With a 200 W amplifier into 8 Ω over 20 m, a 1.5 mm² cable (0.46 Ω) drops about 5.4% of the voltage, costs 0.49 dB and wastes about 10 W, while a 2.5 mm² cable (0.28 Ω) drops 3.3%, costs 0.29 dB and wastes about 6 W. This free calculator gives these numbers for any cable and tells you the smallest standard size for the loss you accept.

Why does a speaker cable lose voltage and power?

Every wire resists the flow of current a little, and a speaker cable carries the large current coming from the amplifier. The resistance of the cable sits in series with the speaker, so part of the amplifier voltage is used up in the wire and turns into heat instead of sound. A long or thin cable loses more, and speakers with low impedance, which draw more current, suffer more. Losses are measured in decibels because that is how we hear them: about 0.5 dB is hard to notice, while more than 1 dB is clearly audible and wastes power. The fix is simple: use a thicker cable, a shorter run, or both.

Made by Luca Martone, sound designer and live sound engineer based in Turin.
For more information, click on the logo below.