bolt Ohm's Law & Power Calculator star

How It's Calculated

Ohm's law states V = I × R — voltage equals current times resistance. Combine it with the power law P = V × I and three more identities fall out: P = I² × R and P = V² / R. Together these five equations link four quantities — voltage, current, resistance and power — so that any two of them fix the other two.

Worked example: a 12 V battery is wired to a 4 Ω resistor. Current follows directly from Ohm's law: I = V / R = 12 / 4 = 3 A. Power follows from P = V × I: 12 × 3 = 36 W. Swap in resistance and current instead — the same 36 W = I² × R = 3² × 4 checks out, and 36 W = V² / R = 144 / 4 confirms it a third way. That triangulation is exactly why the calculator lets you enter whichever two values you actually measured and derives the rest, rather than forcing you to memorize which formula fits which pair.

Every one of the six input pairs maps to a closed-form pair of outputs: given V and I, R = V/I and P = V·I; given V and R, I = V/R and P = V²/R; given V and P, I = P/V and R = V²/P; given I and R, V = I·R and P = I²·R; given I and P, V = P/I and R = P/I²; given R and P, V = √(P·R) and I = √(P/R). The tool runs whichever branch matches the two fields you filled in.

What You Should Know

This calculator is scoped deliberately to DC circuits and purely resistive loads — heaters, incandescent bulbs, simple resistor networks. The moment a circuit involves an inductor, capacitor or an AC motor winding, current and voltage drift out of phase, and a power factor term (cos φ) has to be multiplied into the power equation. Skipping that step is the single most common mistake people make when they try to size a motor or transformer with Ohm's law alone: P = V × I only holds for AC when the load is purely resistive.

• Resistance is temperature-dependent. A cold incandescent filament measures far lower resistance than the same filament glowing at operating temperature, so a resistance reading taken with a multimeter on a cold component can understate the real in-circuit value.
• Tolerance stacks. A resistor marked 100 Ω with 5% tolerance could be anywhere from 95 to 105 Ω, which propagates into every value this calculator derives from it.
• Units matter before you type. Convert milliamps to amps (÷1000) and kilohms to ohms (×1000) first — the calculator does not guess your prefix.

For motors, ballasts and anything with a nameplate power-factor rating, use the dedicated motor current calculator instead — it accounts for power factor and efficiency, the two variables Ohm's law alone cannot see.