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Ohm's Law Calculator

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Enter any two of voltage, current, and resistance - the third is calculated automatically, along with power.

Ohm's Law Calculator

Live
Current (I) =
3 A
Power
36 W
Fill in exactly two values, and leave the one you want to solve for blank.

The formula

V = I x R I = V / R R = V / I Power: P = V x I
Example

A 12V source across a 4Ω resistor: I = 12/4 = 3A, and power dissipated = 12 x 3 = 36W.

Step-by-step guide

  1. Fill in any two values - voltage, current, or resistance - and leave the third blank.
  2. Read the calculated value, along with power, which is derived automatically from voltage and current.

Why Ohm's Law matters

Ohm's Law is the single most fundamental relationship in basic electronics and electrical work - it connects voltage (the electrical "push"), current (the flow of charge), and resistance (opposition to that flow) in a simple, direct way. Nearly every other electrical calculation, from sizing a resistor to diagnosing a circuit fault, ultimately traces back to this relationship. It applies directly to simple resistive circuits; more complex circuits with capacitors, inductors, or AC power need extended versions of these principles.

Common mistakes

Mixing up milliamps and amps - if your current is in mA, divide by 1,000 before using this calculator, or your resistance and power results will be off by a factor of 1,000.
Applying Ohm's Law directly to a non-resistive or nonlinear component (like a diode or LED) - those don't follow a simple V=IR relationship across their full operating range.

Frequently asked questions

Does Ohm's Law work for AC circuits?

For purely resistive AC loads, yes, using RMS voltage and current values. Circuits with capacitors or inductors need impedance (a more general form of resistance that accounts for AC behavior) instead of simple resistance.

What's the difference between resistance and impedance?

Resistance applies to simple resistive elements and DC circuits. Impedance is the AC equivalent, accounting for how capacitors and inductors react differently to changing current, and includes a phase relationship that plain resistance doesn't.

How is power related to Ohm's Law?

Power (P = V x I) isn't technically part of Ohm's Law itself, but combining it with Ohm's Law gives two other useful forms: P = I²R and P = V²/R, both handy depending on which values you already know.

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