A primary factor in choosing car subwoofers is pairing them correctly with an amplifier that can power them safely and at full performance. If you're wiring one subwoofer to one amplifier, the calculation is straightforward. But if you're wiring multiple subwoofers to a single amplifier — or working with a dual voice coil subwoofer — calculating the total effective impedance of the system is a necessary step for both safety and performance.
What Is Impedance (Ohms)?
Ohms are a measure of impedance — resistance to electrical current flow. A larger ohm rating equals a greater load on the amplifier. A higher impedance load results in less amplifier output: an amplifier will output more power at 2 ohms than at 4 ohms.
Most car amplifiers can be safely wired at 4 or 2 ohms. Some are stable down to 1 ohm depending on their design — always check your amplifier's spec sheet before wiring.
Series vs. Parallel Wiring
Series Impedance Calculation
In series wiring, you add the impedance of each voice coil together:
Parallel Impedance Calculation
With subwoofers rated at equal impedances, parallel wiring simplifies to:
Single vs. Dual Voice Coil Subwoofers
If you have a single voice coil (SVC) subwoofer, there's only one wiring option — the SVC impedance is what the amplifier sees. But if your subwoofer has a dual voice coil (DVC), you can wire the two coils in series or parallel, giving you control over the final impedance. This flexibility is one of the biggest advantages of DVC subs. Learn more in our guide: Single vs. Dual Voice Coil Subwoofers.
Real-World Example: Two SVC Subwoofers
Bob has an MTX TN500.1 500W Mono Amplifier (300W @ 4 ohms / 500W @ 2 ohms) paired with one MTX 3512-04 12" 250W 4-ohm subwoofer. He wants to add a second 3512.
With a single 4-ohm sub, the amp delivers 250W — a perfect match for the sub's rating. For two subs Bob needs 2 × 250W = 500W. Wiring the two 4-ohm subs in parallel: 4 ÷ 2 = 2 ohms. The amp delivers exactly 500W at 2 ohms, so each sub gets its full 250W.
Real-World Example: Dual Voice Coil Subwoofers
Bob now wants to use that same TN500.1 with two 55 Series DVC subwoofers rated at 400W RMS each.
Two 5512-44 (dual 4-ohm) subs could be wired at 4 ohms or 1 ohm — 4 ohms won't deliver enough power, and 1 ohm would damage the amp. Not a match.
Two 5512-22 (dual 2-ohm) subs could be wired at 2 ohms or 0.5 ohms. The 2-ohm wiring keeps the amp safe and delivers 500W — not the full 800W the subs could handle, but a safe, reliable, and high-performing system.
This is why understanding impedance before purchasing matters — the right wiring configuration can be the difference between a great system and a mismatched one that damages your equipment.
