How it works
Ohm's law says the current through a resistor equals the voltage across it divided by its resistance. Combined with the power law (P = V × I), any two of the four quantities fix the other two.
Pick the pair you know, enter them, and the calculator returns the remaining values. It is handy for sizing a resistor for an LED, checking how much current a heater element draws, or finding the power a resistor must dissipate.
These relations hold exactly for DC circuits and for purely resistive AC loads using RMS values. Motors, transformers and electronics with reactance need power factor taken into account.
Formula
V = I × R P = V × I = I² × R = V² ÷ R
Questions people ask
What resistor do I need for an LED?
Subtract the LED forward voltage from the supply voltage, then divide by the LED current. For a 5 V supply, 2 V red LED and 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. Choose the next standard value up.
How do I pick the resistor wattage?
Compute P = I² × R for the resistor and choose a part rated for at least twice that, so it runs cool. A resistor dissipating 0.2 W should be at least a 1/2 W part.
Does Ohm's law work for AC?
Yes for purely resistive loads such as heaters and incandescent bulbs, using RMS voltage and current. For inductive or capacitive loads you need impedance instead of resistance, and real power also depends on power factor.