Ohm's Law Calculator
Give any two of voltage, current, resistance or power — get the other two.
How it works
Ohm's law is V = I x R, and power is P = V x I. Between them, any two of the four quantities determine the other two — which is why the four-way "Ohm's law wheel" exists and why every derived form on it is just those two identities rearranged.
Enter the two you know and leave the rest at zero. The useful check: if the power figure surprises you, look at the resistor rating before the circuit does it for you.
Common questions
Which resistor wattage do I need?
At least double the calculated power, as a working rule. A resistor dissipating 0.4 W wants a 1 W part — components run hot at their rating, and hot parts drift and age.
Why does doubling the current quadruple the heat?
Because P = I²R. Power rises with the square of current, so a 2x current is a 4x heat load. This is why undersized wiring fails suddenly rather than gradually.