Enter any two of the four values below — voltage, current, resistance, or power — and the rest solve automatically.
Ohm's Law describes how voltage, current, and resistance relate in an electrical circuit: voltage equals current multiplied by resistance. Push more voltage through a fixed resistance and current rises with it; increase the resistance while holding voltage steady and the current drops. It's the single most-used equation in electronics, which is why it's usually the first thing taught in any electrical course.
Add power (P) to the mix and Ohm's Law expands into a full set of power formulas, since power is simply voltage multiplied by current:
Say a 5V supply drives an LED strip drawing 0.5A. Resistance works out to 5V ÷ 0.5A = 10Ω, and the power dissipated is 5V × 0.5A = 2.5W. That wattage figure matters just as much as the ohms — a resistor or wire rated below 2.5W will overheat and fail even though the resistance math checks out. Always size components for at least double your calculated power.
Ohm's Law only holds for linear, "ohmic" components — plain resistors and wires at a roughly constant temperature. Diodes, LEDs, and transistors are non-linear: their effective resistance changes with the current passing through them, so V = I × R can't predict their behavior directly. For those parts you need the manufacturer's datasheet curves instead of this formula.
Yes, but substitute impedance (Z) for resistance (R). Impedance accounts for the extra phase effects that capacitors and inductors add on top of plain resistance.
Resistors are only manufactured in standard series (E12, E24, etc.), not every possible value. Round your result up to the nearest standard size, then check it against your tolerance needs using the Resistor Color Code Calculator.
Voltage and resistance are usually easiest to measure directly with a multimeter — enter those two above and this calculator solves current and power automatically.
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