Ohm's law calculator
Ohm’s law is two equations — V = I × R and P = V × I — and between them any two of the four quantities give the other two. That is why it is usually drawn as a wheel: twelve rearrangements of the same two relationships, none of which needs memorising if you know the two.
Give any two of voltage, current, resistance and power, and it works out the other two. That is the whole of Ohm’s law plus the power equation.
How to use the ohm's law calculator
Ohm’s law proper is one equation, V = I × R, and the power relationship P = V × I is a separate one. Between them any two of the four quantities determine the other two, which is what the familiar wheel diagram encodes — twelve rearrangements that all reduce to those two equations. Learning to rearrange them beats memorising the wheel.
The law describes ohmic components, which is a real restriction. A resistor is ohmic: double the voltage and the current doubles. A filament lamp is not — its resistance rises sharply as it heats, so a cold bulb draws a large inrush current for a few milliseconds, which is why filament lamps almost always fail at the moment you switch them on rather than during use. Diodes, LEDs, motors and anything semiconductor are all non-ohmic too, and applying the law to them gives a number that is right only at one operating point.
On AC the equations still hold but the quantities change meaning. Voltage and current are RMS values, and P = V × I gives apparent power in volt-amperes rather than real power in watts — the two differ by the power factor, which is 1 for a heater and can be 0.6 or worse for a motor. That is why an inductive load draws more current than its wattage suggests, and why supplies are rated in VA rather than W.
Questions
Any two of the four. The other two follow from V = I × R and P = V × I.