Speaker cable calculator
How much of your amplifier ends up as heat in the wire. Enter the run and the gauge to get loop resistance, power lost, level loss in dB and the damping factor that actually reaches the speaker — plus the thinnest gauge that does the job.
Measure the run one way — the calculator doubles it, because the current has to come back. Set damping factor to 0 to model an ideal amplifier and see what the cable alone costs.
| Gauge | Loop R | Power lost | Level | Longest run |
|---|
Badges apply the 5 % rule at your run length: ok under 5 % of the speaker's impedance, close 5–10 %, over more than 10 %. The last column is the longest run each gauge supports while staying inside the rule.
How this is calculated
A conductor's resistance is R = ρ · L / A — resistivity times length over cross-sectional area. Current has to travel out to the speaker and back, so a run counts twice:
Rloop = 2 · ρ · L / A
That resistance sits in series with the speaker, so the two split the amplifier's output in proportion to their resistance. The share burned in the cable, and the level that survives to the driver, are:
power lost = R / (R + Z) · level = 20·log₁₀( Z / (Z + R) ) dB
Copper is taken at ρ = 1.724 × 10⁻⁸ Ω·m (annealed copper, 20 °C). AWG areas come from the standard definition, d = 0.127 mm × 92(36−n)/39, which reproduces published AWG resistance tables to better than 0.1 %. Real stranded cable measures a few percent higher than solid conductor of the same nominal area, and resistance rises about 0.4 % per °C above 20 °C — so treat the results as a tight lower bound, not a guarantee.
How much loss is too much
The working rule across the industry is to keep loop resistance under 5 % of the speaker's nominal impedance. That costs about 0.4 dB and roughly 5 % of the amplifier's power — inaudible, and cheap to achieve. Under 2 % is the target worth hitting for critical listening or low-impedance speakers; past 10 % you are paying for amplifier power that never reaches the driver.
The raw dB figure understates the problem, though. A speaker's impedance is not the flat number on the box — it swings with frequency, often from 3 Ω to 40 Ω across the band. A resistive cable forms a voltage divider with that swinging impedance, so a too-thin cable does not just turn everything down, it tilts the frequency response by a few tenths of a dB wherever the impedance dips or peaks. That interaction, not the average level, is what people are actually hearing when a heavier cable "sounds different".
Damping factor, honestly
Damping factor is the ratio of the load impedance to everything driving it: DF = Z / (Ramp + Rcable). Manufacturers quote it into 8 Ω, so an amp rated DF 200 has an output impedance of 8/200 = 0.04 Ω. Add a cable and the number collapses fast — 10 m of 16 AWG contributes about 0.26 Ω, six times the amplifier's own contribution, and DF drops from 200 to around 26.
That sounds alarming and mostly isn't. Above a DF of roughly 20 the differences stop being measurable at the driver, because the woofer's own voice-coil resistance — typically 6–7 Ω on an "8 Ω" driver — is already in series with the motor and dominates the electrical damping. The cable's contribution matters when it becomes comparable to that, which is the same place the 5 % rule already flags. Use the damping figure as a sanity check, not a target to maximise.
FAQ
What gauge do I need for a 50 ft / 15 m run?
Into 8 Ω, 14 AWG (2.5 mm²) is comfortably inside the 5 % rule and 16 AWG is borderline. Into 4 Ω the budget halves, so 12 AWG (4 mm²) is the sensible choice. Set the run length above and the last table column answers this for every gauge at once.
Do 4 Ω speakers really need heavier wire?
Yes, and it's the single most common mistake. The rule is a percentage of impedance, so halving the impedance halves the resistance you're allowed — the same cable that's fine on an 8 Ω speaker is twice as lossy on a 4 Ω one. Two 8 Ω speakers wired in parallel present 4 Ω and land in exactly this trap.
Is copper-clad aluminium (CCA) wire a problem?
It's aluminium wire with a thin copper skin, and it carries roughly 61 % of copper's conductivity — about 1.64× the resistance for the same gauge, or two AWG numbers' worth. CCA sold as "16 AWG" performs like 18 AWG copper. It's not dangerous at speaker-level currents and it's much cheaper, but budget for the extra thickness: switch the conductor dropdown above and watch what happens to the numbers. CCA also work-hardens and breaks more readily at screw terminals.
Is thicker wire always better?
Better electrically, yes, but with sharp diminishing returns — going from 2 % of the speaker's impedance down to 1 % buys you 0.09 dB nobody can hear, at double the copper cost, and very heavy cable is awkward to terminate and can lever binding posts loose. Get inside the rule, then stop.
Does the wire's length or its gauge matter more?
They trade off exactly: resistance is proportional to length and inversely proportional to area, so doubling the run and stepping up three AWG numbers cancel out almost precisely. If a run is long enough to need awkward cable, moving the amplifier is usually cheaper than upgrading the wire.
Do exotic cables — silver, litz, special geometries — help?
Resistance is the one property in this circuit with a measurable, audible effect, and it's set by metal, area and length. Silver is about 7 % better than copper per unit area — one AWG step is worth three times that, for a fraction of the price. Spend on gauge and good terminations; there's nothing else in a speaker cable to buy.
What about 70 V / 100 V distributed lines?
Different regime entirely. Transformers step the voltage up at the amp and back down at each speaker, which raises the line impedance by orders of magnitude — so the same 5 % rule against a much larger number lets thin wire run hundreds of meters. This calculator models direct low-impedance connection, which is what home, studio and stage-monitor systems use.
Does any of this change how loud the system plays?
Barely. A cable sitting exactly on the 5 % limit costs 0.42 dB at the driver. Reaching a clearly audible −3 dB takes a loop resistance above 40 % of the speaker's impedance — around 50 m (165 ft) of 20 AWG into 8 Ω. Use the SPL calculator to see how little level a given power change buys, and the decibel converter to move between ratios and dB directly.