Zerdly

Speaker Cable Power Loss Calculator

Estimate how much amplifier power is lost as heat in the speaker cable. Enter the run length, the speaker impedance and the cable's resistance per metre, and the tool returns the total loop resistance and the loss as a percentage of the delivered power.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the one-way cable run length in metres (the distance from amp to speaker).
  2. Enter the speaker's nominal impedance in ohms.
  3. Optionally set the cable resistance per metre — 0.0126 Ω/m for 1.5 mm² or 0.0075 Ω/m for 2.5 mm² — then read the loop resistance and power loss.

How it works

Current flows out and back, so the loop resistance is twice the run: R_loop = 2 × length × resistance per metre. The cable and speaker form a series divider, so the fraction of power dropped in the cable is R_loop ÷ (R_loop + speaker impedance), expressed as a percentage. Lower cable resistance and shorter runs shrink R_loop and the loss.

Worked example

Worked example. A 20 m run of 1.5 mm² cable (0.0126 Ω/m) into an 8 Ω speaker: R_loop = 2 × 20 × 0.0126 = 0.504 Ω, and loss = 0.504 ÷ (0.504 + 8) × 100 = 5.93%. That is a little above the 5% comfort target, so heavier cable would help.

Common mistakes

  • Using the one-way length only — the current travels the full loop, so the resistance must be doubled.
  • Ignoring cable gauge: a thin 0.75 mm² lead on a long run can lose several dB into a 4 Ω speaker.
  • Forgetting that a 4 Ω speaker suffers roughly double the percentage loss of an 8 Ω speaker for the same cable.

Frequently asked questions

How much cable loss is acceptable?

Keep it under about 0.5 dB, which is roughly 11% power loss, and ideally under 5%. Small losses are inaudible in level but still lower the damping factor, which can loosen the bass.

Does thicker cable really make a difference?

Yes. Going from 1.5 mm² to 2.5 mm² cuts the resistance per metre by about 40%, which nearly halves the loss on a long run. For long runs to low-impedance speakers, gauge matters most.

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