Ohm's Law
Calculate voltage, current, resistance, or power using Ohm's Law. Select what you want to find, then enter any two known values.
What do you want to calculate?
Enter Known Values(enter any 2 values)
Ohm's Law Formulas
VVoltage
ICurrent
RResistance
PPower
Car Audio Examples
Example 1: Amplifier Current Draw
A 3000W RMS amplifier at 80% efficiency running at 14.4V:
• Actual power draw = 3000W ÷ 0.80 = 3750W
• Current draw (I) = P ÷ V = 3750W ÷ 14.4V = 260.4A
Example 2: Voice Coil Resistance Check
Testing a subwoofer with a multimeter: 1V applied, 0.25A measured:
• Resistance (R) = V ÷ I = 1V ÷ 0.25A = 4Ω
• This is a 4-ohm voice coil
Example 3: Power at the Sub
Measuring 40V AC across a 2Ω subwoofer load:
• Power (P) = V² ÷ R = 40² ÷ 2 = 1600 ÷ 2 = 800W RMS
Reactive Loads vs. Pure Resistance
Speakers are reactive loads — their impedance changes with frequency due to inductance (Le), back-EMF, and enclosure tuning. The nominal impedance you enter here is a baseline reference. At resonance, impedance can spike 5-10× above nominal. At the enclosure tuning frequency, "box rise" typically doubles the effective load. Use these calculations as a starting point, then verify with measured impedance data for precision applications.
Practical Notes
- •Multimeter readings show DC resistance (Re), which runs about 15-20% below the speaker's rated impedance
- •Voltage fluctuates in vehicles: around 12.6V with engine off, 14.4V running, sometimes 16V+ with high-output alternators
- •Amp efficiency affects draw: Class D typically converts 80-90% to output, Class AB sits around 50-65%
- •Stick with RMS for real-world power math—peak ratings look impressive but don't reflect continuous output