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Ohm’s Law Calculator

V = I × R ; P = V × I V, A, Ω, W

Ohm's law applies to DC and to purely resistive AC circuits. For AC circuits with inductance or capacitance (motors, transformers, most real loads), replace resistance with impedance (Z) and use apparent power (S) rather than real power (P). This calculator assumes an ideal resistive load.

What is Ohm’s law?

Ohm’s law states that the current flowing through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance. In its usual form: V = I × R. This one equation, together with the power equation P = V × I, is the foundation of practically every DC and purely resistive AC calculation you’ll ever do.

How to use this calculator

Pick which two values you already know from the “I know” dropdown — Voltage & Current, Voltage & Resistance, Current & Power, or any other pair. Enter those two values, and the calculator returns all four quantities (V, I, R, P) with the exact formula it used for each derivation. Units auto-format: type 500 mA and the result may come back showing 12 V, 24 Ω, 6 W.

The formulas at a glance

All twelve rearrangements of Ohm’s law and the power equation are just algebra applied to the two base formulas:

  • V = I × R — voltage from current and resistance
  • I = V / R — current from voltage and resistance
  • R = V / I — resistance from voltage and current
  • P = V × I — power from voltage and current
  • P = I² × R — power from current and resistance
  • P = V² / R — power from voltage and resistance
  • V = √(P × R) — voltage from power and resistance
  • I = √(P / R) — current from power and resistance

A worked example

You have a 12 V DC supply and a load that draws 500 mA. What is the load’s resistance, and how much power does it dissipate?

  • R = V / I = 12 V ÷ 0.5 A = 24 Ω
  • P = V × I = 12 V × 0.5 A = 6 W

If you pick a resistor for this load, it needs to be at least a 1/4 W part — and preferably 1/2 W or 1 W for headroom, because a resistor operated close to its rated power will run hot and drift.

When Ohm’s law does NOT apply cleanly

Ohm’s law is an idealization. It holds well for pure resistors, incandescent bulbs (approximately, once warmed up), and heaters. It does not hold directly for:

  • AC circuits with inductance or capacitance — motors, transformers, coils, capacitor loads. Use impedance (Z) instead of resistance, and remember that real vs apparent power (W vs VA) differ by the power factor.
  • Semiconductor junctions — LEDs, diodes, transistors. These have non-linear V-I curves; use the manufacturer’s datasheet.
  • Batteries under load — the terminal voltage sags as current draw increases due to internal resistance.

For most everyday DC electronics and resistive AC loads, though, Ohm’s law is exact enough to design and troubleshoot with.

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