Ohm's Law Calculator

Ohm's Law is the foundation of electrical circuit analysis. It states that voltage equals current times resistance (V = I × R). Knowing any two quantities lets you calculate the third. This calculator also shows power, since P = V × I.

• LED resistor sizing: Calculate what resistor to use with an LED to limit current to a safe level.
• Circuit design: Verify that voltage and current specs are met for components in a circuit.
• Troubleshooting: Measure two values with a multimeter and calculate the third to diagnose faults.
• Power dissipation: Check that a resistor can handle the power it will dissipate to avoid overheating.

1. 1Enter any two of voltage (V), current (I), and resistance (R).
2. 2Leave the unknown value as 0.
3. 3The calculator detects which value is missing and applies the appropriate Ohm's Law variant.
4. 4Power (watts) is also calculated from the three values.

• Entering all three values: The calculator solves for the unknown value from any two known ones. Enter only the two you know and leave the unknown empty or at zero. Entering all three overconstrains the system.
• Confusing resistance and impedance: Ohm's Law V = IR applies to DC circuits and purely resistive AC circuits. Impedance (Z) extends this to capacitors and inductors in AC circuits. Use resistance (R) for basic DC calculations.
• Ignoring wire resistance in long runs: For short cables, wire resistance is negligible. For long cable runs (e.g., 30 m of thin speaker wire), the resistance adds up and can measurably reduce voltage at the end of the run.
• Using peak voltage when RMS is needed: Mains electricity is described in RMS volts (120V or 240V). Peak voltage is higher (~170V or 340V). Use RMS values for power calculations unless you are specifically working with peak values.

• Calculate power in the same step: Power (W) = V × I = I²R = V²/R. The calculator shows power alongside the main result. Use it to check if a resistor or device will exceed its power rating before wiring.
• Use for LED resistor sizing: To find the correct series resistor for an LED: subtract LED forward voltage from supply voltage, then divide by LED current. For a 5V supply, 2V LED, 20 mA: (5-2) / 0.02 = 150 Ω.
• Apply to speaker wiring: Speaker impedance is the load resistance. Amplifier output voltage divided by speaker impedance gives current draw. Use this to check whether an amplifier can safely drive multiple speakers in parallel.
• Verify fuse sizing: A fuse protects a circuit by blowing at a rated current. Calculate expected current from voltage and load resistance, then choose a fuse rated slightly above normal operation current (typically 125% of expected).