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?

VoltageV

—V

CurrentI

ResistanceR

PowerP

V = I × RCurrent × Resistance

V = P ÷ IPower ÷ Current

V = √(P × R)√(Power × Resistance)

I = V ÷ RVoltage ÷ Resistance

I = P ÷ VPower ÷ Voltage

I = √(P ÷ R)√(Power ÷ Resistance)

R = V ÷ IVoltage ÷ Current

R = V² ÷ PVoltage² ÷ Power

R = P ÷ I²Power ÷ Current²

P = V × IVoltage × Current

P = V² ÷ RVoltage² ÷ Resistance

P = I² × RCurrent² × Resistance

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

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

Measuring 40V AC across a 2Ω subwoofer load:

• Power (P) = V² ÷ R = 40² ÷ 2 = 1600 ÷ 2 = 800W RMS

•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